A hedge fund is a privately pooled investment fund that aggregates capital primarily from accredited individuals or institutional investors and employs a wide array of aggressive strategies—often involving leverage, derivatives, short selling, and non-traditional assets—to pursue absolute returns uncorrelated with broader market movements.[1][2][3] Unlike mutual funds, hedge funds operate with minimal regulatory oversight, face fewer disclosure requirements, and typically charge high performance-based fees structured as "2 and 20" (2% management fee plus 20% of profits), enabling managers greater flexibility but also amplifying risks.[4][5]The modern hedge fund traces its origins to 1949, when Alfred Winslow Jones launched the first such vehicle, combining long positions in undervalued stocks with short sales to hedge market risk, thereby pioneering the "hedged" approach that gave the industry its name.[6] The sector expanded significantly in the 1990s amid deregulation and a bull market, attracting star managers from traditional finance and swelling assets under management to trillions of dollars globally by the 2000s.[6] Strategies diversified to include global macro bets, event-driven arbitrage, quantitative models, and activist interventions, with notable successes such as George Soros's 1992 short against the British pound yielding over $1 billion in profits.[7]Hedge funds have achieved outsized returns in specific niches, contributing to market efficiency through liquidity provision and price discovery, yet empirical data reveals average long-term performance often trails passive indices like the S&P 500 after fees, prompting scrutiny of their value proposition.[8] Defining controversies include the 1998 collapse of Long-Term Capital Management, which nearly triggered systemic failure due to excessive leverage and required Federal Reserve intervention; insider trading scandals at firms like SAC Capital; and amplified roles in amplifying volatility during events like the 2008 financial crisis.[9][7] Recent critiques highlight persistent underperformance, opaque fee structures favoring managers over investors, and limited transparency, fueling debates over whether their risks justify purported benefits amid growing alternatives like low-cost ETFs.[8][9]
Science and technology
Chemistry
Hydrogen fluoride (HF) is a diatomic molecule composed of one hydrogen atom covalently bonded to one fluorine atom, with the gas-phase structure being linear (H–F) and a bond length of approximately 0.92 Å.[10] In the liquid and solid states, HF molecules associate into polymeric chains or zig-zag structures through strong hydrogen bonding, arising from fluorine's high electronegativity (4.0 on the Pauling scale), which polarizes the H–F bond and enables F···H–F interactions stronger than those in other hydrogen halides.[11] This hydrogen bonding accounts for HF's anomalously high boiling point of 19.5 °C and melting point of −83.6 °C relative to HCl, HBr, and HI, despite its low molecular weight of 20.01 g/mol.[10] The vapor density is 1.86 times that of air, and anhydrous HF is a colorless, fuming gas or low-boiling liquid with a density of about 1.00 g/cm³ at 0 °C.[12]Chemically, anhydrous HF is highly reactive and corrosive, acting as a powerful fluorinating agent due to the strength of the H–F bond (bond dissociation energy of 565 kJ/mol, the highest among hydrogen halides).[13] It reacts vigorously with many metals to form metal fluorides and hydrogen gas, such as 2HF + Mg → MgF₂ + H₂, and with oxides or hydroxides to yield fluorides.[13] A distinctive reaction is its ability to etch glass and silica, dissolving SiO₂ via SiO₂ + 4HF → SiF₄ + 2H₂O, producing volatile silicon tetrafluoride; this occurs because fluoride ions complex with silicon, unlike other acids.[14] In aqueous solution (hydrofluoric acid), HF behaves as a weak acid with pKₐ = 3.17 at 25 °C, partially dissociating due to the strong H–F bond and high hydration energy of F⁻, though it forms complexes like [HF₂]⁻ in concentrated solutions.[11]Industrial production of anhydrous HF involves heating acid-grade fluorspar (CaF₂, ≥97% purity) with concentrated sulfuric acid in a rotary kiln or distillation setup: CaF₂ + H₂SO₄ → CaSO₄ + 2HF (g), followed by purification via distillation to remove water and impurities, yielding 99.98% purity HF.[13] This process, developed commercially in the early 20th century, accounts for global production exceeding 1 million metric tons annually, primarily for fluorochemical synthesis.[15] HF's reactivity necessitates storage in materials resistant to fluorination, such as polytetrafluoroethylene or steel alloys, as it attacks most common container materials except noble metals and fluoropolymers.[16]
Biology and medicine
Hafnium exhibits no known biological role in living organisms and is not essential for any enzymatic or physiological processes.[17][18] Studies indicate that hafnium does not participate in biochemical systems, with limited natural occurrence in biological tissues due to its geochemical rarity and chemical inertness.[19] In microbial contexts, certain proteins like the ferric ion-binding protein from Neisseria gonorrhoeae can bind hafnium ions in vitro, but this has no established functional significance in vivo.[19]Toxicity assessments reveal hafnium and its compounds generally possess low acute and chronic toxicity in humans, with no reported industrial poisonings or symptoms from dietary exposure.[20][21] Elemental hafnium metal is non-toxic and poorly absorbed via ingestion, though fine powders may pose inhalation risks as irritants to respiratory mucosa.[17]Hafnium salts can cause mild irritation to eyes, skin, and mucous membranes, and animal studies link high-dose exposure to liver damage, but human lethal dose estimates (e.g., LD50 for hafniumchloride in rats at 2400 mg/kg orally) suggest a wide safety margin.[22][23] No evidence supports hafnium as a carcinogen or mutagen under typical environmental exposures.[21]In medicine, hafnium finds applications primarily through engineered nanomaterials rather than as a bulk element. Hafnium oxide nanoparticles, such as NBTXR3, are designed for intratumoral injection to enhance radiotherapy efficacy; these high-Z particles absorb X-rays and emit photoelectrons and Auger electrons, amplifying local ionizing radiation damage to cancer cells while sparing surrounding healthy tissue.[24] Clinical trials, including phase II/III studies for soft tissue sarcomas, have demonstrated improved tumor control rates with NBTXR3 combined with external beam radiation, with acceptable safety profiles (e.g., common adverse events limited to injection-site reactions).[25] Hafnium-based nanostructures also serve as contrast agents in computed tomography (CT) imaging and for liquid biopsies in cancer diagnostics, leveraging their radiopacity and biocompatibility.[18]Hafnium alloys and coatings show promise in biomedical implants, particularly for orthopedic and dental applications. When applied as a surface modification to titanium implants, hafnium enhances osseointegration by promoting osteoblast proliferation and differentiation, outperforming uncoated titanium in preclinical models of bone-implant interfaces.[26] Its corrosion resistance and radiopacity aid in durable prosthetic designs, with in vitro studies confirming low cytotoxicity and favorable cell adhesion.[27] Ongoing research explores hafnium's integration into scaffolds for tissue engineering, though long-term human data remain limited as of 2024.[18]
Other uses in science and technology
In telecommunications, high frequency (HF) denotes the electromagnetic spectrum band spanning 3 to 30 MHz, enabling long-distance communication through ionospheric reflection, known as skywavepropagation. This mechanism allows signals to bounce off the ionosphere, facilitating beyond-line-of-sight (BLOS) links over thousands of kilometers without reliance on satellites or repeaters.[28][29] HF systems remain vital in scenarios with disrupted infrastructure, such as remote operations or during solar activity that affects propagation predictability.[30]Military and aviation sectors extensively employ HF radio for global connectivity; for instance, the U.S. Air Force utilizes the High Frequency Global Communications System (HF-GCS) for aircraft-to-ground voice and data transmission, including pilot telephone patching.[31]Humanitarian logistics also leverage HF for coordination in austere environments, where its bandwidth supports voice, email, and low-data-rate transfers over HF bands.[29] Despite vulnerabilities to atmospheric interference, advancements like digital signal processing have enhanced HF reliability, sustaining its role as a resilient alternative to satellite communications.[32]In radio-frequency identification (RFID), HF operates at 13.56 MHz for short-range applications, such as smart cards, access control, and inventory tracking of items with high water or metal content, outperforming ultra-high frequency (UHF) in such cases due to reduced interference.[33]The chemical element hafnium (Hf), with its high melting point and corrosion resistance, supports advanced technological applications beyond basic chemistry. In nuclear reactors, hafnium's exceptional neutron absorption cross-section—over 500 times that of zirconium—enables its use in control rods to regulate fission reactions efficiently.[34][35]Hafnium alloys enhance turbine blades and rocket nozzles in aerospace, enduring temperatures exceeding 2000°C while maintaining structural integrity under extreme conditions.[36]Semiconductor manufacturing incorporates hafnium(IV) oxide (HfO₂) as a high-k dielectric material in metal-oxide-semiconductor field-effect transistors (MOSFETs), replacing silicon dioxide to minimize gate leakage in nanoscale chips and enabling continued transistor scaling per Moore's law trends as of 2024.[36][37] These properties stem from hafnium's electronic structure, providing a dielectric constant around 25, far superior to silicon dioxide's 3.9.[35]
Medicine
Heart failure
Heart failure is a pathophysiological state in which the heart cannot generate sufficient cardiac output to meet the body's tissue perfusion needs or can only do so at elevated filling pressures, often resulting from structural or functional cardiac abnormalities. It manifests as a syndrome rather than a single disease entity, encompassing reduced ejection fraction (HFrEF, where left ventricular ejection fraction is ≤40%) or preserved ejection fraction (HFpEF, where ejection fraction is >50% with evidence of diastolic dysfunction). The condition arises primarily from impaired myocardial contractility, increased afterload, or volume overload, leading to compensatory mechanisms like neurohormonal activation that eventually exacerbate progression.[38][39]Major causes include coronary artery disease, which accounts for approximately 60-70% of cases through ischemic damage to myocardium; hypertension, promoting left ventricular hypertrophy and stiffness; and cardiomyopathies such as dilated or hypertrophic variants. Other contributors encompass valvular heart disease, arrhythmias like atrial fibrillation, congenital defects, and toxic exposures including alcohol or chemotherapy agents. Diabetes mellitus and chronic kidney disease synergistically heighten risk by inducing microvascular damage and fibrosis. Risk factors are modifiable (e.g., smoking, obesity) or non-modifiable (e.g., age >65 years, male sex for HFrEF).[40][38]Symptoms typically develop insidiously in chronic forms but can present acutely with decompensation, including exertional dyspnea due to pulmonary congestion, peripheral edema from sodium retention, and fatigue from low perfusion. Orthopnea, paroxysmal nocturnal dyspnea, and jugular venous distension signal elevated left- or right-sided pressures, while ascites and hepatomegaly indicate right heart involvement. In advanced stages, cachexia, renal hypoperfusion leading to azotemia, and hepatic congestion occur. Objective signs include cardiomegaly, third heart sound (S3 gallop), and crackles on auscultation.[41][42]Diagnosis relies on clinical history, physical examination, and confirmatory tests: natriuretic peptides (e.g., BNP >100 pg/mL or NT-proBNP >300 pg/mL suggest high likelihood); echocardiography to assess ejection fraction, chamber sizes, and diastolic function; and electrocardiography to detect ischemia or arrhythmias. Biomarkers like troponins aid in ruling out acute coronary syndromes, while chest radiography reveals congestion. Invasive hemodynamics via catheterization are reserved for ambiguous cases or transplant evaluation. Classification uses New York Heart Association (NYHA) functional classes I-IV based on symptom limitation or ACC/AHA stages A-D reflecting risk to advanced disease.[38][43]In the United States, heart failure affects 6.7 million adults aged ≥20 years as of 2024, with projections reaching 8.7 million by 2030 due to aging populations and rising comorbidities like obesity and diabetes; globally, prevalence exceeds 64 million. Incidence has increased, particularly among younger adults (≤45 years), with mortality rates rising since 2012—now accounting for 45% of cardiovascular deaths—and a 5-year survival rate of approximately 50% post-diagnosis, worse in HFpEF subsets. Disparities persist, with higher rates in Black Americans attributable to socioeconomic factors and hypertension prevalence.[44][45][46]Treatment targets symptom relief, mortality reduction, and hospitalization prevention, stratified by ejection fraction. For HFrEF, guideline-directed medical therapy (GDMT) per 2024 ACC consensus includes quadruple therapy: angiotensin receptor-neprilysin inhibitors (e.g., sacubitril/valsartan, reducing mortality by 20% vs. ACE inhibitors); beta-blockers (e.g., carvedilol); mineralocorticoid receptor antagonists (e.g., spironolactone); and SGLT2 inhibitors (e.g., dapagliflozin, lowering HF events by 25-30% across ejections). Diuretics manage congestion, while devices like implantable cardioverter-defibrillators prevent sudden death in select patients (e.g., EF ≤35%). HFpEF management emphasizes diuretics, SGLT2 inhibitors, and comorbidity control (e.g., blood pressure <130/80 mmHg), with limited options due to heterogeneous pathophysiology. Advanced therapies include left ventricular assist devices or transplantation for refractory cases, with cardiac rehabilitation improving outcomes via exercise and diet (e.g., low-sodium DASH).[47][48][49]Prognosis varies by etiology and timeliness of intervention; early GDMT halves mortality risk, but decompensated episodes carry 20-30% one-year mortality. Monitoring via implantable sensors (e.g., pulmonary artery pressure) reduces hospitalizations by 30% in trials. Causal emphasis on reversing drivers—like revascularization for ischemia—yields better causal outcomes than symptomatic palliation alone, underscoring the need for etiological precision over generic labeling.[50][38]
Organizations
Charitable organizations
Humanity First is an international nongovernmental organization founded in 1995, dedicated to humanitarian aid through immediate disaster response and long-term development initiatives in sectors including water access, healthcare, education, and food security. It has conducted operations in over 50 countries, responding to crises such as earthquakes, floods, and conflicts, while implementing sustainable projects like building schools and medical clinics. As of 2023, the organization reported transforming millions of lives via programs in Europe, Africa, Asia, and the Americas.[51][52]The Hispanic Federation, established in 1990, functions as a national nonprofit umbrella organization supporting Latino communities in the United States by granting funds to over 780 member nonprofits and delivering direct programs in education, health, immigration services, civic engagement, and economic development. Operating across 43 states and territories including Puerto Rico, it has distributed hundreds of millions in aid since inception, with a focus on disaster relief following events like Hurricanes Maria in 2017 and Fiona in 2022. Charity Navigator has awarded it a four-star rating for accountability and transparency for six consecutive years through 2023.[53][54]Health First Foundation, with origins tied to the 1937 establishment of Holmes Regional Medical Center in Florida, supports the not-for-profit Health First healthcare network by fundraising for uncompensated patient care, facility upgrades, advanced medical equipment, and community wellness initiatives such as hospice services and caregiver training. As the largest provider of charitable healthcare in Brevard County, it invests millions annually from donor contributions, operating on thin 1-3% margins while serving Medicare, Medicaid, and uninsured patients across four hospitals.[55]The Heart Failure Patient Foundation is a U.S.-based nonprofit focused on heart failure awareness, patient education, and support networks for affected individuals, families, and caregivers, including advocacy for research participation and access to clinical studies. It provides resources to mitigate isolation and improve management of the condition, which impacts over 6 million Americans as of recent estimates.[56]
Political organizations
The Hispanic Federation (HF), established on October 9, 1990, in New York City, functions as a nonprofit membership organization dedicated to advancing Hispanic communities via local, state, and national advocacy, including political empowerment efforts such as voter registration drives and civic mobilization. Since inception, HF has registered and engaged millions of Latino voters through nonpartisan campaigns, partnering with over 50 organizations in initiatives like the 2024 Latino voter outreach that emphasized election impacts on community issues, and launching toolkits for the 2025 elections in collaboration with groups like F.Y. Eye and the New York Latino Civic Engagement Coalition.[57][58][59] Its policy work targets civil rights, justice, and equity, including a 2025 federal policy series addressing immigration, economic conditions, and deportations affecting Latinos, though critics note its alignment with progressive priorities may reflect institutional biases in nonprofit advocacy networks.[60][61]The HF Sinclair Political Action Committee (DINO PAC), linked to HF Sinclair Corporation—a Dallas-based energy firm formed in 2022 from the merger of HollyFrontier and Sinclair Oil—operates as a connected PAC to influence federal elections through corporate contributions. In the 2023-2024 cycle, the PAC raised $345,794 from individual donors and employees, directing funds primarily to Republican candidates and committees, with $316,947 in total contributions amid the company's $430,000 lobbying expenditures on energy policy.[62] This reflects standard industry practices for PACs in regulated sectors like oil refining, where such entities bundle employee donations to support pro-business legislation without direct corporate treasury involvement.[63]
People
Scientists and inventors
Dirk Coster and George de Hevesy discovered hafnium in 1923 while analyzing zirconium ores from Norway and Greenland using X-ray spectroscopy at the University of Copenhagen.[64][65] Coster, a Dutchphysicist, identified the element's characteristic X-ray lines, which matched predictions for atomic number 72, while de Hevesy, a Hungarian-Swedish chemist, oversaw the chemical separation from zirconium impurities.[66][67] Their work resolved a prior false claim of discovery by French chemist Georges Urbain, who had misidentified a supposed element "celtium" in 1911.[65]De Hevesy, who earned the 1943 Nobel Prize in Chemistry for pioneering the use of isotopes as tracers in chemistry, applied similar separation techniques to isolate hafnium, highlighting its geochemical association with zirconium at ratios up to 1:150 in minerals like zircon. Coster's spectroscopic expertise built on earlier predictions of element 72's properties by Niels Bohr, who anticipated its zirconium-like behavior based on quantum theory.[65]In 1925, Dutch chemists Anton Eduard van Arkel and Jan Hendrik de Boer developed the van Arkel-de Boer process, a vapor-phase purification method involving the thermal decomposition of hafnium tetraiodide to yield high-purity hafnium metal crystals.[66] This innovation enabled practical applications of hafnium, such as in alloys for high-temperature environments, by removing zirconium contaminants that degrade neutron absorption properties.[66] Their technique, still used today, underscored hafnium's strategic value in nuclear reactor control rods due to its high thermal neutron capture cross-section of approximately 104 barns.[66]
Public figures and activists
Former U.S. Vice PresidentDick Cheney was diagnosed with advanced congestive heart failure in October 2010, following a heart attack earlier that month and prior cardiac events including a heart attack in 2000 and quadruple bypass surgery in 1988. He received a left ventricular assist device (LVAD) implant shortly after diagnosis to support his failing heart and underwent a successful heart transplant on March 24, 2012, at age 71, making him one of the oldest patients to receive such a procedure at the time. Cheney has shared details of his management of the condition, including reliance on medical devices and transplant, to highlight advancements in heart failure treatment.[68]Actress Elizabeth Taylor experienced recurrent congestive heart failure in her later years, compounded by prior conditions like atherosclerosis and pneumonia; she died from the condition on March 23, 2011, at age 79. Taylor's public battles with heart issues, including hospitalizations for CHF exacerbations, drew attention to the disease's impact on aging celebrities.[69][68]Patient advocates have also emerged in heart failure awareness efforts. Audwin Helton, a survivor of severe heart failure treated at Norton Heart & Vascular Institute, has conducted public education on cardiac risk factors and early intervention since his recovery around 2023, emphasizing lifestyle modifications and medical adherence.[70] The Global Heart Hub's Heart Failure Patient Council, comprising international patient organizations, advocates for improved diagnostics, access to therapies, and policy changes as of 2025, representing thousands of affected individuals globally.[71]
Arts and entertainment
Film and media
Heyday Films, often abbreviated as HF in its logo, is a British film production company founded by David Heyman in London in 1997.[72] The company gained prominence for producing all eight Harry Potter films between 2001 and 2011, as well as other titles including Gravity (2013) and Paddington (2014).[73] Its logo features a stylized "HF" monogram drawn over the text "HEYDAY FILMS".[74]Hot Fuzz (2007), sometimes referenced by the initials HF in production details and fan discussions, is an action comedy film directed by Edgar Wright and co-written by Wright and Simon Pegg.[75] Starring Pegg as Nicholas Angel and Nick Frost as Danny Butterman, the film satirizes buddy cop tropes and rural policing in England, grossing over $80 million worldwide on a $12 million budget. It forms part of the "Three Flavours Cornetto" trilogy alongside Shaun of the Dead (2004) and The World's End (2013).[76] Film-specific elements, such as vehicle markings labeled "HF" on a Subaru Impreza used by the protagonist, directly nod to the title's abbreviation.[75][77]HF Productions, rebranded as HFP in recent years, is an organization operating film festivals and events across multiple countries to promote independent cinema and social impact narratives.[78] It hosts events like the Arctic Film Festival and Rome Independent Cinema Festival, emphasizing training, mentorship, and diversity in filmmaking.[79][80]High Frequency (1997) is a Canadian TV movie involving amateur radio operators who witness a murder via high-frequency transmissions, highlighting ham radio's role in the plot.[81]
Music and performing arts
Hudobný fond, commonly abbreviated as HF, is a public-law cultural institution in Slovakia focused on fostering musical composition, performance, and dissemination. Established by Act No. 13/1993 Coll. on artistic funds enacted by the National Council of the Slovak Republic, it serves as a national entity to support creators and interpreters in the musical domain. Headquartered at Medená 29 in Bratislava, the organization manages funding allocations through a governing council that reviews and approves applications based on defined support principles, with guidelines last updated and effective as of February 21, 2025.[82][83]The fund's core activities encompass granting financial support for creative projects in music, including stipends for performers—such as those targeted at classical music interpreters for 2025—and facilitating the publication and distribution of sheet music and compact discs via its integrated e-shop. It also handles aspects of music archiving and promotion, operating under a framework that prioritizes direct aid to artistic endeavors while adhering to annual calls for proposals, like project grants for 2026. These efforts aim to sustain professional musical activity amid evolving legislative and cultural priorities in Slovakia.[84][85][86]
Business and finance
Corporate entities
In Iceland, "hf." is the standard abbreviation for hlutafélag, a public limited liability company whose shares may be publicly traded and which requires a minimum share capital of ISK 4,000,000 divided into at least two parts.[87] This entity type is commonly used by Icelandic corporations, with many appending "hf." to their names to indicate their legal form under the Icelandic Companies Act.[88]HF Sinclair Corporation is an independent energy company focused on refining and marketing petroleum products, including gasoline, diesel, and renewable diesel, with operations centered in the mid-continent, southwestern, and Rocky Mountain regions of the United States.[89] Formed on August 1, 2022, via the merger of HollyFrontier Corporation and Sinclair Oil Corporation, it operates seven refineries with a combined crude oil processing capacity of approximately 678,000 barrels per day.[90]HF Foods Group Inc. is a marketer and distributor of fresh produce, frozen and dry foods, seafood, and non-food products, primarily serving Asian restaurants and supermarkets across the United States.[91] Headquartered in Las Vegas, Nevada, the company sources specialty items from international and domestic suppliers, leveraging a network of distribution centers to reach a diverse customer base rooted in cultural understanding of Asian cuisines.[92]HFF, Inc. was a full-service commercial real estatefinancial intermediary providing debt placement, investment sales, and advisory services to property owners and investors.[93] Established as a leading player in capital markets transactions, it generated over $650 million in revenue in 2018 before being acquired by Jones Lang LaSalle Incorporated (JLL) on July 1, 2019, integrating its operations into JLL's platform.[93]
Sports
Teams and competitions
In handball, "HF" frequently appears as part of Swedish club names, abbreviating handbollsförening (handball club), with teams competing in domestic leagues like the Handbollsligan (top tier) and Allsvenskan (second tier), as well as EHF-organized European events such as the EHF European League.[94]HF Karlskrona fields a men's team that participated in the 2025-26 EHF European League, drawing on a roster blending domestic and international players for competitive matches across Europe.[94]Ystads IF HF maintains active men's and women's squads in Swedish leagues and has featured in EHF competitions, leveraging group stage formats to challenge regional opponents.[95]Lugi HF, based in Lund, operates professional men's and women's teams in the Allsvenskan for the 2025-26 season, emphasizing structured seasons with promotion/relegation pathways.[96]In American youth sports, HF United Youth Football & Cheer operates tackle football and cheer programs in Flossmoor, Illinois, for participants aged 6-14, focusing on skill fundamentals, safety protocols, and team-based competitions within regional youth leagues.[97]Homewood-Flossmoor High School Vikings, commonly abbreviated HF, compete in Illinois High School Association-sanctioned events across sports like American football, where their varsity team clinched a playoff spot with a 29-10 win over Naperville Central Redhawks on October 25, 2025.[98]
Other uses
Slang and communications
In internet slang, particularly within online gaming communities such as those for multiplayer online battle arena (MOBA) games like Dota 2 or League of Legends, "HF" abbreviates "have fun," serving as a courteous phrase exchanged between players at the start or conclusion of matches to promote sportsmanship.[99][100] This usage often pairs with "GL" for "good luck," forming "GL HF" to encapsulate mutual well-wishes without implying guaranteed outcomes.[101] Its adoption traces to early 2000s forums and chat systems in competitive gaming, where brevity in text-based interactions favors acronyms over full phrases.[99]Less commonly, "HF" appears in niche online contexts, such as hookup apps, where it may euphemistically denote "high fun" referring to methamphetamine use during encounters, though this interpretation lacks broad recognition outside specific subcultures.[102]In telecommunications, HF designates high frequency, the International Telecommunication Union (ITU)-defined radio band spanning 3 to 30 MHz (wavelengths of 10 to 100 meters), enabling long-distance signal propagation through ionospheric reflection, or skywave, which supports applications like maritime, aviation, and military communications beyond line-of-sight limitations.[103][104] This band experiences diurnal and seasonal variations due to solar activity affecting the ionosphere, rendering it unreliable for short-range but ideal for global reach without infrastructure dependency.[28] HF systems have seen renewed interest in modern scenarios, such as disaster response, where satellite alternatives fail, as evidenced by deployments in remote oceanic aviation routes requiring continuous coverage.[32][105]