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Hyperplane arrangement
A hyperplane arrangement is a finite collection of hyperplanes, codimension-one affine subspaces, in a space like R^d, studied for how they cut space into regions.
Hyperpolarization (biology)
Hyperpolarization (biology): shift of a cell's membrane potential to a more negative value, caused by K+ efflux or Cl- influx; underlies IPSPs, the afterhyperpolarization, and hyperpolarization-activated HCN/I_h currents.
Hyperpolarized carbon-13 pyruvate
Hyperpolarized carbon-13 pyruvate is a nonradioactive metabolic imaging agent, magnetically polarized and injected intravenously, imaged with MRI to measure real-time metabolism in tumors and heart tissue.
Hyperreal number
Hyperreal numbers, also called nonstandard real numbers or hyperreals, are an ordered field extending the real numbers with infinitesimals, used in Abraham Robinson's nonstandard analysis.
Hypersimplex
A hypersimplex Δd(k) is a convex polytope generalizing the simplex, the slice of the unit cube [0, 1]^d by a hyperplane summing to k, central in polytope and matroid theory.
Hypersonic flow
Hypersonic flow is the flow of gas around a body moving faster than Mach 5, a regime dominated by strong shock waves, high-temperature gas effects, and thick boundary layers.
Hyperspectral change detection
Hyperspectral change detection compares two co-registered hyperspectral images of the same area, acquired at different times, to locate surface changes, with uses in disaster monitoring, land survey, and military reconnaissance.
Hyperspectral imaging
Hyperspectral imaging, also called imaging spectroscopy, records a scene in hundreds of narrow wavelength bands, letting each pixel's spectrum identify materials in agriculture, geology, food safety, and medicine.
Hyperspherical coordinates
Hyperspherical coordinates are a coordinate system for several interacting particles in which the hyperradius measures overall size and hyperangles describe shape; they treat all rearrangement channels equally in quantum reactive scattering.
Hypertriton lifetime puzzle
The hypertriton lifetime puzzle is the disagreement between heavy-ion collision measurements of the hypertriton's lifetime, which range from values matching the free Λ hyperon down to roughly half of it.
Hyperuniform states of active matter
Hyperuniform states of active matter are nonequilibrium assemblies of self-driven agents in which large-scale density fluctuations are suppressed to crystal-like levels, seen in circle swimmers, algae, and active field theories.
Hypervalent iodine chemistry
Hypervalent iodine chemistry is the branch of chemistry dealing with iodine compounds in +3 and +5 oxidation states, used as selective, metal-free oxidants that resemble transition metals.
Hypervalent iodine(III) reagents
Hypervalent iodine(III) reagents, the λ3-iodanes, are trivalent iodine compounds acting as powerful electrophilic oxidants and group-transfer reagents in organic synthesis, including PIDA, iodonium salts, and benziodoxoles used in C–H activation.
Hypervalent organotellurium compounds
Hypervalent organotellurium compounds are organotellurium molecules in which tellurium formally exceeds the octet rule; they include σ-telluranes, telluroxanes, and cyclic telluranes, and are more thermally stable than sulfur or selenium analogs.
Hypervelocity stars
Hypervelocity stars are stars fast enough to escape the Milky Way's gravity entirely, exceeding Galactic escape velocity; the term was coined in 1988 and the first was found in 2005.
Hypobromous acid
Hypobromous acid (HOBr) is a weak bromine oxoacid formed in water when bromide is oxidized, the active biocidal form of bromine disinfectants and a reactive oxidant in the human body.
Hypocenter
A hypocenter, also called the focus, is the point within the Earth where an earthquake rupture starts; the epicenter lies directly above, and focal depth strongly affects shaking intensity.
Hypochlorite
Hypochlorite is the oxyanion of chlorine in the +1 oxidation state, OCl⁻, and the family of salts containing it, including sodium hypochlorite, the active ingredient in bleach.
Hypochlorous acid
Hypochlorous acid (HOCl, also written HClO) is a weak acid that exists only in water, formed when chlorine dissolves, and is the active microbicidal form of chlorine in disinfection.
Hypoglycin A
Hypoglycin A is a heat-stable, toxic amino acid found in unripe ackee fruit and maple seeds, causing Jamaican vomiting sickness in people and atypical myopathy in horses.
Hypolimnion
The hypolimnion is the cold, dense bottom layer of a stratified lake, lying beneath the thermocline and cut off from atmospheric oxygen during summer stratification.
Hypophosphorous acid
Hypophosphorous acid, systematically named phosphinic acid (H3PO2), is a monoprotic phosphorus oxyacid and strong reducing agent, used mainly as a 50% aqueous solution for electroless nickel plating and hypophosphite production.
Hyporheic zone
The hyporheic zone is the porous region of a stream bed and banks where surface water and shallow groundwater mix, a subsurface environment distinct from both.
Hypotenuse
The hypotenuse is the side of a right triangle opposite the right angle, and the longest side, given by the Pythagorean theorem and central to sine and cosine.
Hypothesis
A hypothesis (Greek: ὑπόθεσις) is a specific, tentative statement about natural phenomena, made before a scientific process, that guides what data are collected and is supported or refuted by evidence.
Hypothetico-deductive model
The hypothetico-deductive model is an account of scientific method in which a scientist deduces predictions from a hypothesis and tests them, often presented as the scientific method.
Hypoxanthine
Hypoxanthine, also called 6-hydroxypurine, is a purine nucleobase central to purine metabolism: it is the base of inosine, an intermediate in adenosine breakdown, and rises when cells are deprived of oxygen.
Hypromellose
Hypromellose, also called hydroxypropyl methylcellulose (HPMC), is a water-soluble cellulose ether that controls drug release in matrix tablets, lubricates artificial tears, and thickens food and cement mortars.
Hypsithermal
The Hypsithermal, also known as the Altithermal or Holocene Climatic Optimum, was a warm interval of the mid-Holocene, about 9000 to 5000 years ago, caused by orbital forcing.
Hypusine
Hypusine, a modified lysine amino acid made from spermidine, occurs only in the translation factor eIF5A, whose activity in protein synthesis it enables; it was discovered in 1971.