Hubble's law describes the relationship between a galaxy's distance and its recessional velocity. In its simplest form, the law says that more distant galaxies generally recede from us faster than nearer galaxies.
Edwin Hubble published observational evidence for this relationship in 1929, using galaxy distances and redshift measurements. The relationship is commonly written as v = H₀D, where v is recessional velocity, D is distance, and H₀ is the Hubble constant. A galaxy's redshift provides an estimate of how quickly it is receding, while distance must be established through astronomical distance indicators.
The law is evidence that the universe is expanding, not that galaxies are simply flying away from one central point through otherwise static space. Cosmic expansion stretches space itself. The relationship is most reliable on large scales; nearby galaxies can have substantial peculiar velocities caused by local gravitational interactions. The law is also often called the Hubble–Lemaître law, recognizing Georges Lemaître's earlier theoretical and observational work.