(physics) the constant of proportionality (symbol h), relating the energy and frequency of a photon (6.626 × 10⁻³⁴ joule-seconds); also related to the indeterminacy limit in the Heisenberg uncertainty principle; the quantum of action

(physics) the constant of proportionality in the equation of Coulomb's law; numerical value depends on the units used

(physics) any of a set of physical quantities that play a fundamental role in basic theories

(physics) a universal constant, R, that appears in the ideal gas law, PV=nRT, derived from two fundamental constants, the Boltzmann constant and Avogadro's number, R=N_Ak.

Alternative spelling of Hubble constant

(astronomy) A number that expresses the rate of expansion of the universe; the velocity at which a distant object is receding from the Earth divided by its distance; its reciprocal determines the age of the universe.

(physics) A numerical constant used in determining the electrostatic energy of each ion in a crystal.

(physics) A universal scaling factor for all spectroscopic transitions.

The amount of incoming solar radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays.

(mathematics) A function whose value is the same for all the elements of its domain.