Entry 22 · the alphabet of motion
The speed at which an animated value is changing per unit time.
CSS @keyframes · velocity
The long entry
Velocity is the first derivative of position. When you spec an animation by 'velocity-out' rather than 'duration', you are telling the engine to move at a constant rate of change rather than fitting the move into a fixed window. Springs, by definition, are velocity-driven: you set physics, not time.