In an elastic collision, the kinetic energy of the system is conserved, while in an inelastic collision, some of the kinetic energy is lost as heat, sound, or deformation of the objects. The coefficient of restitution is a measure of the elasticity of the collision and is defined as the ratio of the relative velocity of separation to the relative velocity of approach between the objects. For a perfectly elastic collision, the coefficient of restitution is equal to one, while for a perfectly inelastic collision, it is equal to zero.