Recessional Velocity is a term used to describe the rate at which an object is moving away, typically from Earth.
[edit] Application to Cosmology
This term is generally only used in reference to distant Galaxies. The most common reason for the use of this term is Hubble's Law, which states that the redshift is proportional to the distance between an inertial observer and a distant galaxy. The redshift is usually interpreted as due to Recessional Velocity which can be calculated according to the formula:
where H0 is the Hubble constant, D is the intervening distance, and v is the Recessional Velocity, generally measured in km/s.
The recessional velocity of a galaxy is usually calculated from the redshift observed in its emitted electromagnetic radiation. The distance to the galaxy is then estimated using Hubble's Law.