When two objects collide and bounce away from each other, they tend to lose some of their energy in the collision, and the rebound velocity between the two objects is therefore less than the initial velocity between them. This is why objects that are dropped will sooner or later stop bouncing. The elasticity of the collision can be indicated by e, the coefficient of restitution, which is defined as the relative speed after the collision divided by the relative speed before the collision:
A perfectly elastic collision would therefore have a coefficient of restitution equal to one; an elastic ``super ball'' is a good example of an object whose coefficient of restitution in many collisions is often close to one.