Physics material
A reusable bundle of surface properties — chiefly friction and bounce — that you stick on a collider instead of setting the numbers per object.
Two numbers go a long way toward making a surface feel right: how much it slows sliding contact (friction) and how much speed it keeps in a bounce (restitution). Collecting them into a named material — Unity's PhysicMaterial, Unreal's Physical Material — lets ice, rubber and stone be reused across the whole game.
A subtlety is what happens when two materials meet. The engine has a combine rule — average, minimum, maximum or multiply — that decides the pair's friction and bounce. Forget it and a rubber ball on ice ends up as sluggish as the ice or as bouncy as the rubber, depending on which rule applies.