Hubble's constant, Hubble constant, Hubble's parameter, Hubble parameter(noun)

(cosmology) the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer; the Hubble constant is not actually a constant, but is regarded as measuring the expansion rate today

Hubble constant(Noun)

a number the 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 gives the age of the universe