Easing
How a movement speeds up and slows down between its start and its end.
See it in SVG path animator. Switch Easing between linear and ease-in-out, then press play.
In plain words
Real things rarely start or stop at full speed. A bicycle pulls away slowly and slows down before it stops. Easing gives animation the same feel by changing the speed along the way. Ease-in starts slow, ease-out ends slow, and ease-in-out does both. Linear keeps one speed from start to end, which often looks stiff. The steeper an easing curve, the faster the thing is moving at that moment.