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Grothendieck spectral sequence
The Grothendieck spectral sequence is a tool in homological algebra that computes the derived functors of a composite of two functors; Grothendieck introduced it in 1957.
Grothendieck topology
A Grothendieck topology is a structure on a category specifying which families of morphisms count as coverings, generalizing open covers; a category with one is called a site.
Grothendieck topos
A Grothendieck topos is a category equivalent to sheaves of sets on a site, introduced by Alexander Grothendieck in SGA 4 (1963–1964) as the framework for étale cohomology.
Grothendieck's Galois theory
Grothendieck's Galois theory is Alexander Grothendieck's axiomatic reformulation of Galois theory, covering space theory, and the algebraic fundamental group, given in SGA 1 (1960/61); it characterizes Galois categories via a fiber functor.
Grotthuss mechanism
The Grotthuss mechanism, proposed by Theodor von Grotthuss in 1805, moves protons through hydrogen-bonded networks by successive hops, explaining their anomalously high mobility in water and enabling conduction in fuel-cell membranes.
Ground level enhancement
A ground level enhancement (GLE) is a sudden rise in counts at ground-based cosmic ray detectors, chiefly neutron monitors, caused by solar particles energetic enough to reach the ground.
Ground state
In quantum mechanics, the ground state is the allowed state of lowest energy, labeled ψ₀; its energy need not be zero, as the harmonic oscillator's zero-point energy shows.
Ground truth
Ground truth is the ideally correct value or label against which a measurement or model output is judged, in measurement science and machine learning alike.
Ground wave
A ground wave is a radio wave that travels along or near the Earth's surface without reflection from the ionosphere, letting AM stations reach beyond the visual horizon.
Ground-penetrating radar
Ground-penetrating radar (GPR), or georadar, is a nondestructive geophysical method that transmits radio-frequency pulses into the ground and records reflections to map buried features and their depths.
Ground-state cooling of a micromechanical oscillator
Ground-state cooling of a micromechanical oscillator is the cooling of a microfabricated mechanical resonator below one phonon of thermal motion, chiefly by sideband cooling, first demonstrated at NIST in 2011.
Grounded theory
Grounded theory is a qualitative research methodology that generates theories inductively from systematically collected data. Barney Glaser and Anselm Strauss developed it in the 1960s.
Groundhog Day
Groundhog Day, also called Daks Day, is a North American folk custom on February 2 in which a groundhog's shadow is said to predict winter's length.
Groundwater
Groundwater, also called subsurface water or phreatic water, is water held in cracks and spaces of rock and sediment below the Earth's surface, the largest store of liquid freshwater.
Groundwater age dating
Groundwater age dating is the estimation of the time since water recharged an aquifer, inferred from dissolved tracers like CFCs, tritium, and carbon-14 rather than measured directly.
Groundwater discharge
Groundwater discharge is the outflow of water from the saturated zone back to the land surface or to streams, lakes, wetlands, and, in coastal areas, the ocean floor.
Groundwater model
A groundwater model, also known as a groundwater flow model, is a computational method that predicts hydraulic heads, flows, and solute concentrations in aquifers, most often with the USGS MODFLOW code.
Groundwater pollution
Groundwater pollution, also called contaminated groundwater, is the contamination of underground aquifers by pesticides, fertilizers, sewage, and industrial waste, which threatens the drinking water of half the U.S. population.
Groundwater protection
Groundwater protection: preventing aquifer contamination through vulnerability mapping, protection zones, land-use controls, and SDWA, EU, and managed recharge programs worldwide.
Groundwater recharge
Groundwater recharge, also called deep percolation or aquifer recharge, is the downward flow of water from the surface to the water table; managed recharge deliberately adds water to aquifers for storage and recovery.
Group (mathematics)
In mathematics, a group is a set with a single operation that is associative and has an identity element and inverses, with examples including integers under addition.
Group (periodic table)
A group of the periodic table is one of the 18 numbered vertical columns of elements, also called families, whose members share the same number of valence electrons.
Group 11 element
A group 11 element, also called the copper or gold group, is one of four chemical elements: copper, silver, gold, and synthetic roentgenium, with copper, silver, and gold known as the coinage metals.
Group 3 element
Group 3 of the periodic table contains scandium and yttrium, with its lower members disputed: lanthanum and actinium traditionally, or lutetium and lawrencium, and IUPAC found no objective criterion.
Group action
A group action, also called an action of a group, is a way for a group to operate on a set, defined as a homomorphism into its permutations.
Group algebra of a locally compact group
The group algebra of a locally compact group G is the Banach -algebra L¹(G), integrable functions under convolution, the basic object linking Banach algebra theory to harmonic analysis.
Group cohomology
Group cohomology is a branch of homological algebra that assigns abelian groups Hn(G, A) to a group G, measuring how G acts on a module.
Group concept mapping
Group concept mapping is a mixed-methods participatory technique in which a group brainstorms, sorts, and rates statements, and multivariate analysis turns those judgments into a concept map.
Group contribution method
The group contribution method is a chemical estimation technique that predicts properties like boiling points and critical properties by summing the contributions of a molecule's structural fragments.
Group delay and phase delay
Group delay and phase delay, together called delay distortion, are two frequency-dependent measures of how much a linear system or medium delays a signal's components and envelope.