Science

A critical point marks the disappearance of a phase distinction in physics, or a zero or undefined derivative in calculus.

Cryo-electron microscopy images rapidly frozen specimens with electron beams to reveal molecular structures and cellular organization in three dimensions.

Crystallography studies the atomic arrangement, symmetry, and properties of crystals using mathematical analysis and experimental techniques.

Cybernetics is the interdisciplinary study of communication, feedback, and regulation in living organisms, machines, and social systems.

Cytokinesis is the physical division of a cell’s cytoplasm into daughter cells, coordinated with chromosome segregation through mechanisms that differ among organisms.

The cytoskeleton is a dynamic network of protein filaments that organizes cells, provides mechanical support, and enables movement, intracellular transport, and division.

Dark energy is the unknown component invoked to explain the accelerating expansion of the universe, commonly modeled as a cosmological constant.

Dark matter is an unseen component of the universe inferred from gravitational effects, whose physical nature remains unidentified.

Deep inelastic scattering probes the quark and gluon structure of hadrons through high-energy lepton collisions with large momentum transfer.

Degeneracy pressure is the resistance to compression produced by the quantum-state occupancy restrictions of fermions, persisting even at zero temperature.

Density is mass per unit volume, describing how much matter occupies a given space and how that quantity varies with material and physical conditions.

A density matrix represents a quantum state, encompassing pure states, statistical mixtures, and the reduced states of entangled systems.

Diamagnetism is a magnetic response that opposes an applied field, giving materials negative magnetic susceptibility and weak repulsion from stronger-field regions.

A dielectric is a material that responds to an electric field primarily through polarization rather than sustained electrical conduction.

Diffraction is the spreading and interference of waves caused by apertures, obstacles, or spatial structure, underlying optical resolution and many methods of structural analysis.

Diffusion is the spreading of particles through random motion, producing net transport down concentration gradients under ordinary conditions.

The Dirac equation is a relativistic equation for spin-½ particles that explains electron spin and provides the foundation for describing fermions and antiparticles in quantum field theory.

Optical dispersion is the frequency dependence of light propagation, producing effects such as color separation, wavelength-dependent delay, and pulse distortion.

The divergence theorem equates the outward flux through a region’s boundary with the integral of a vector field’s divergence over its interior.

DNA is the nucleic acid that stores hereditary information in cellular organisms, providing templates for replication and instructions for RNA production.

DNA ligases are enzymes that seal breaks in DNA strands, supporting replication, repair, and laboratory DNA assembly.

DNA methylation is a chemical modification of DNA bases that influences genome regulation, cellular identity, and protection against foreign DNA.

DNA mismatch repair corrects replication-associated base-pairing errors and small insertion–deletion loops, preserving genome stability.

DNA replication is the semiconservative process by which cells copy their DNA before division, using complementary templates and coordinated enzymes.

Ecology is the scientific study of interactions among organisms and their environment, including the processes governing distribution, abundance, and ecosystem functioning.

A field-theoretic framework that describes physics at a specified scale through a systematic expansion, encoding unresolved shorter-distance effects in interaction coefficients.

Effective population size is the size of an idealized population that would experience the same genetic drift, inbreeding, or ancestral processes as a real population.

The Einstein field equations describe how spacetime geometry responds to matter and energy, forming the dynamical foundation of general relativity.

Electric charge is a conserved property of matter that determines its electromagnetic interactions and occurs in positive and negative forms.

Electric current is the rate of net electric charge flow through a surface, measured in amperes and carried by charged particles.

An electric dipole is a separation of positive and negative charge, characterized by a vector dipole moment that determines its field and interaction with external fields.

An electric field is a vector field that describes the electric force per unit charge at each point in space and time.

Electric potential is electric potential energy per unit charge, describing electrostatic fields through a scalar quantity whose differences determine energy changes.

An electrical conductor is a material containing mobile charge carriers that allow electric current to flow under an applied electric field.

Electrical resistance measures the opposition of a material, component, or circuit to electric current, expressed as the ratio of voltage to current.

Electrochemistry studies the relationships between electrical phenomena and chemical processes, including charge transfer, energy conversion, and reactions at electrode interfaces.

Electromagnetic induction is the production of an electromotive force through changing magnetic flux or the motion of a conductor in a magnetic field.

Electromagnetic radiation transports energy and momentum through space as electromagnetic waves, with quantum interactions described in terms of photons.

The electromagnetic spectrum encompasses all electromagnetic radiation, ordered by wavelength, frequency, or photon energy, from radio waves to gamma rays.

Electromagnetism studies electric and magnetic fields, their interactions with charged matter, and the radiation they produce.

An electron is an elementary particle with negative electric charge, central to atomic structure, chemical bonding, and electrical conduction.

Electron density describes the spatial distribution of electrons, expressed as their expected number per unit volume.

An electron hole is an unoccupied electronic state that behaves as a positively charged quasiparticle, especially in semiconductor valence bands.

An electron microscope uses accelerated electrons to image and analyze structures at scales ranging from cellular components to individual atoms.

A membrane-associated system of electron carriers that couples electron transfer to ion gradients, supporting ATP production in respiration and photosynthesis.

A particle–antiparticle reaction in which an electron and a positron are converted into photons or other particles, conserving total energy and momentum.

Electronegativity measures an atom’s ability to attract electrons in a chemical bond and helps explain bond polarity and periodic trends.

Electronic band structure describes the allowed energies of electrons in a crystalline solid as functions of crystal momentum, providing a basis for understanding electrical and optical properties.

The electronvolt is a unit of energy equal to exactly 1.602176634 × 10⁻¹⁹ joules, defined by the energy gained by an electron accelerated through one volt.

Electrostatics studies stationary electric charges, their fields and potentials, and their interactions with conducting and insulating materials.

The electroweak interaction unifies electromagnetic and weak processes through a gauge theory whose symmetry is broken by the Higgs field.

The elementary charge is the positive magnitude of an electron’s charge and an exact defining constant of the International System of Units.

An enantiomer is one of two stereoisomers that are non-superposable mirror images of each other, with potentially different behavior in chiral environments.

The endoplasmic reticulum is a continuous membrane network in eukaryotic cells that produces and processes proteins and lipids, stores calcium, and coordinates intracellular transport.

The endosymbiotic theory explains the bacterial origins of mitochondria and plastids through long-term integration within ancestral host cells.

Energy is a conserved physical quantity, measured in joules. It can be passed from one system to another and changed between forms, which lets it do work or produce heat.

Energy quantization is the restriction of a physical system’s measurable energies to discrete values, rather than an uninterrupted range.

An enhancer is a DNA regulatory element that increases transcription of a target gene, often in a cell-type-specific or signal-dependent manner.

Enrico Fermi was an Italian-American physicist who developed quantum statistics and beta-decay theory and led the first controlled, self-sustaining nuclear chain reaction.

Enthalpy is a thermodynamic state function combining internal energy with pressure–volume energy, used to describe heat transfer, reactions, and fluid flow.