The coefficient of restitution, also known as the rebound or elasticity coefficient, is a measure of energy lost or conserved during a collision between two objects. In physics, this term describes the ratio of the relative velocities of two colliding objects before and after the collision. In engineering, this term refers to the energy transfer ratio from an object's potential energy to kinetic energy and back. Other synonyms for the coefficient of restitution that are frequently used include the coefficient of elasticity, the impact restitution, or simply the restitution coefficient. Regardless of which term is used, the coefficient of restitution is a critical concept in physics and engineering and is widely used to describe the behavior of colliding objects.