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Correlation coefficient
A correlation coefficient, or coefficient of correlation, is a number between −1 and +1 that measures the linear relationship between two variables, best known as Pearson's r.
Correlation dimension
The correlation dimension, denoted D2, is a fractal dimension introduced by Peter Grassberger and Itamar Procaccia in 1983 to characterize strange attractors, estimated via the Grassberger–Procaccia algorithm.
Correlation does not imply causation
Correlation does not imply causation is a principle of scientific reasoning holding that an association between variables does not alone establish cause, which requires evidence such as a randomized experiment.
Correlation function (physics)
A correlation function in physics is a statistical average of a product of physical quantities at different points in space or time, measuring how fluctuations influence one another.
Correlation function (statistical mechanics)
In statistical mechanics, a correlation function is the thermal average of products of fluctuating quantities, such as spins or particle densities, measuring how fluctuations at different points relate.
Correlation length
The correlation length is the distance over which fluctuations of a thermodynamic variable, such as density or magnetization, stay correlated; its divergence defines continuous phase transitions.
Correlation matrix
A correlation matrix is a square, symmetric matrix whose entries give the correlation coefficient between each pair of variables, with ones on the diagonal and off-diagonal values between −1 and 1.
Correlation spectroscopy
Correlation spectroscopy (COSY) is a two-dimensional NMR experiment, introduced in 1976 by Aue, Bartholdi, and Ernst, whose cross-peaks identify coupled protons for molecular structure assignment.
Correlative microscopy
Correlative microscopy integrates two or more microscopy techniques on the same sample, combining their strengths; its most common form is correlative light and electron microscopy (CLEM).
Correspondence analysis
Correspondence analysis is an exploratory multivariate statistical method that maps the associations in a contingency table, plotting its row and column categories in a single low-dimensional plot.
Correspondence principle
The correspondence principle, introduced by Niels Bohr around 1920, states that quantum transitions correspond to harmonic components of classical motion; it guided Heisenberg's 1925 matrix mechanics, though textbooks now use the name differently.
Corrida de Cori Volcanic Field
Corrida de Cori Volcanic Field, a late Cenozoic volcano chain on the Chile-Argentina border at 25.1°S, 68.3°W; its youngest center is Cerro Escorial (5,451 m), source of the ca. 460 ka Escorial ignimbrite.
Corroboration (epistemology)
Corroboration, in Karl Popper's epistemology, is the appraisal a scientific theory earns by passing severe tests, a report of past performance rather than the probability the theory is true.
Corrosion
Corrosion, also called metal corrosion, is the gradual chemical or electrochemical deterioration of a material, most often a metal, on contact with water, oxygen, salt, and other environmental agents.
Corsi–Rosenthal Box
The Corsi–Rosenthal box, also called a CR box, is a do-it-yourself air cleaner built from MERV-13 filters and a box fan, developed in 2020 by Richard Corsi and Jim Rosenthal.
Cortisone Acetate
Cortisone acetate is the acetate ester of cortisone, a glucocorticoid used as an anti-inflammatory, immunosuppressive, and hormone-replacement drug; an inactive precursor, it is converted by the liver to cortisol.
Corundum
Corundum is a crystalline form of aluminum oxide (Al₂O₃) and the mineral that, with trace coloring elements, produces ruby and sapphire; it ranks 9 on the Mohs scale.
Cory Dean
Cory R. Dean is an experimental condensed matter physicist and professor of physics at Columbia University known for research on two-dimensional materials, moiré quantum matter, and twisted bilayer graphene.
Coset
A coset, left coset or right coset, is a translate gH or Hg of a subgroup H inside a group G; equal-sized cosets partition G, yielding Lagrange's theorem.
Cosine similarity
Cosine similarity is a measure of similarity between two nonzero vectors, the cosine of the angle between them; it runs from 1 for the same direction to -1 for opposite.
COSINE-100
COSINE-100 was a dark matter experiment at the Yangyang Underground Laboratory in South Korea that tested the DAMA/LIBRA claim using 106 kg of sodium iodide crystals.
COSINUS (dark matter experiment)
COSINUS is a dark matter experiment at the Gran Sasso laboratory in Italy using sodium iodide crystals to cross-check the DAMA/LIBRA claim, with its first run in 2026.
Cosmic Background Imager
The Cosmic Background Imager (CBI) was a 13-element radio interferometer built by Caltech that operated on Chile's Chajnantor Plateau from 1999 to 2008, measuring cosmic microwave background temperature and polarization.
Cosmic birefringence
Cosmic birefringence is the rotation of light's polarization plane as it crosses the universe, sought in the cosmic microwave background as a sign of parity-violating physics.
Cosmic censorship hypothesis
The cosmic censorship hypothesis, or cosmic censorship conjecture, is a conjecture in general relativity proposed by Roger Penrose in 1969 that singularities should stay hidden inside black holes.
Cosmic chronometers
Cosmic chronometers are passively evolving galaxies whose differential stellar ages measure the expansion rate H(z) directly at each redshift, a method proposed by Jimenez and Loeb in 2002.
Cosmic coincidence problem
The cosmic coincidence problem, also called the "why now?" problem, asks why dark energy and matter have comparable densities today despite evolving differently as the universe expands.
Cosmic dipole
The cosmic dipole is a large-scale sky anisotropy seen in the cosmic microwave background and in distant quasars, whose mismatch, the cosmic dipole anomaly, exceeds 5σ.
Cosmic distance ladder
The cosmic distance ladder, or extragalactic distance scale, is the chain of overlapping methods that measures astronomical distances step by step, from parallax of nearby stars through Cepheid variables to Type Ia supernovae in distant galaxies.
Cosmic dust
Cosmic dust, also called astrophysical dust or extraterrestrial dust, consists of tiny solid particles in interstellar space that dim starlight and form the raw material of planets.