Restitution
The coefficient of restitution e: how much of its speed an impact keeps — the measure of how bouncy something is.
It is the ratio of the relative speed along the normal after the impact to the speed before it. e = 1 is perfectly elastic: no energy is lost, a superball that rebounds to the same height every time. e = 0 is perfectly inelastic: the bodies stick and all normal speed vanishes. Everything in between is reality. The relation goes back to Isaac Newton's experiments with colliding balls.
In an engine each pair of materials gets an e, combined (min, max or average) when two surfaces meet, and collision response scales the bounce impulse by it. A trap: naive restitution injects energy at resting contacts, so a stack of crates starts to shiver — which is why most engines apply a threshold that zeroes the bounce below a certain speed.