恢复系数（coefficient of restitution）衡量两个物体在碰撞后的反弹程度。假若恢复系数为1，则此碰撞为弹性碰撞；假若恢复系数小于1而大于或等于0，则此碰撞为非弹性碰撞；假若恢复系数为0，则此碰撞为完全非弹性碰撞，两个物体黏贴在一起。

恢复系数是两个碰撞物体之间的共同性质。但是，时常在文献中，恢复系数会被表现为单独物体所具有的内秉性质，而只字不提这物体到底是与哪个物体相互碰撞。在这状况里，第二个物体被假定为完美弹性刚体。

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.