Squash and stretch
The animation principle of letting an object compress and extend as it moves, to convey weight, speed and life.
A rigid bouncing ball looks dead. Squash it flat at the moment of impact and stretch it out on the way up and down — classically preserving volume, so what it loses in height it gains in width — and the brain reads material and force at once. The principle is the first and most fundamental of Disney's twelve, set out by Frank Thomas and Ollie Johnston in The Illusion of Life (1981).
In games it is achieved with scale animation, rigged skeletons or shaders, usually layered on top of an interpolated motion with a soft easing curve. Dosed right it gives a character weight; overdone, everything turns rubbery and cartoonish, which suits some styles and ruins others.