Coefficient of restitution

In classical mechanics, the coefficient of restitution (COR, also denoted by e), is a measure of the springiness of collisions between two surfaces. Newton's law of restitution states that the relative speed of separation of two surfaces immediately after a collision is directly proportional to the relative speed of approach of the surfaces immediately before the collision, the constant of proportionality called the coefficient of restitution, a dimensionless parameter that is a property of a pair of surfaces.

Source: Wikipedia — Coefficient of restitution (CC BY-SA 4.0)