Secondary motion
Movement not driven directly by a character's intent but following from it — hair swishing, a cape trailing, a pouch bouncing a moment after the body stops.
One of Disney's twelve principles of animation (Thomas and Johnston, The Illusion of Life, 1981). The point is that a character never moves in a single rigid piece: when the torso halts, the hair, the coat and everything loose keep going a fraction of a second longer. Without it, motion reads as stiff and robotic.
In games it is usually computed at runtime rather than hand-keyed: spring bones, cloth simulation, or a simple lag-and-overshoot on a transform. Cheap liveliness — but the secret is damping. Too little and it feels dead; too much and everything wobbles like jelly.