Restitution, or rather the coefficient of restitution is a number that describes the bounciness of an object. Unit restitution means that the collision is elastic, which means that energy is preserved in the collision. A low coefficient means that the collision is inelastic, thus meaning that energy is lost in heat and other processes during the collision. Because of that loss, the object has got less energy to go up again.

The formula for restitution is the following:

Where V_12f are the velocities after the bounce, and

V_12i the velocities before the bounce.

The restituion cannot be defined for a single material, but for a material or object pair. In Phun, a simple model is used:

Where R1 and R2 are the restitution coefficents of the two colliding geometries.

List of illustrative restitution values^[1] for objects bouncing against a hard surface ("floor"):

Material	Restitution
range golf ball	0.86
tennis ball	0.71
billiard ball	0.8
hand ball	0.75
wooden ball	0.6
steel ball bearing	0.6
glass marble	0.66
ball of rubber bands	0.83
hollow, hard plastic ball	0.69

References

1. ↑ Common restitution values at The Physics Factbook