WordNet (r) 3.0 (2006):

Hubble constant
    n 1: (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 [syn: Hubble's constant, Hubble constant,
         Hubble's parameter, Hubble parameter]