Recessional velocity
Recessional velocity is the rate at which an object is moving away, typically from Earth.
Application to cosmology
Recessional velocity is most pertinent to distant galaxies, which (due to Hubble's Law) redshift proportionally to their distance from the Earth. The redshift is usually interpreted as due to recessional velocity, which can be calculated according to the formula:
where 
is the Hubble constant, 
is the intervening distance, and 
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. Since the distance is constantly increasing, the recession velocity is also increasing in magnitude. Therefore, "recession velocity" is a misnomer. More properly, this phenomena should be referred to as the recession acceleration.