Easing

Also called: easing curve, timing function, ease-in, ease-out, ease-in-out.

How an animation speeds up and slows down over its duration. Ease-out, a fast start with a gentle stop, is the safe default for things appearing on screen.

Every ball takes the same 1.2 seconds. Only the easing is different.

ease-out 0s

ease-out 0.3s

ease-out 0.6s

ease-out 1.2s

Start

Say it in a prompt

Use ease-out, cubic-bezier(0.16, 1, 0.3, 1), at 300ms for elements entering the screen, and ease-in at 200ms for elements leaving.

Seen on

Stripe: Content in the product dialog rises and fades in over 0.75s on a curve that starts fast and settles slowly.
Linear: The site's CSS names its motion curves (ease-out-quad, ease-in-out-cubic, ease-out-expo) and reuses them everywhere.

You might describe it as

animation that starts fast and slows down at the end
movement feels robotic and not smooth
speed curve of the motion

Not to be confused with

Spring
Easing is the whole family of speed curves; spring is the one that overshoots and settles.
Stagger
Easing shapes one item's speed; stagger sets the delay between several items.