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
Source: WordNetIt maps bumps and wiggles in spacetime—the cosmic microwave background, or CMB—and reflects conditions in the early universe as set by Hubble's constant. Source: Internet