The coefficient of restitution (COR) is a measure of the "restitution" of a collision between two objects: how much of the kinetic energy remains for the objects to rebound from one another vs. how much is lost as heat, or work done deforming the objects.
The coefficient, e is defined as the ratio of relative speeds after and before an impact, taken along the line of the impact:
Coefficient of restitution ( e ) = Relative velocity after collision
Relative velocity before collision
Alternatively, this may be expressed as: Relative velocity of separation = e × Relative velocity of approach.