Glossary · Animation
Keyframe
A moment in an animation where the look of an element is defined; between two keyframes, the browser calculates the movement.
In detail
In an animation you define only the important moments (the element appears, reaches its place, disappears), and the browser fills in what is in between. Those moments are the keyframes.
In CSS they are written with @keyframes. With JavaScript, the Web Animations API (element.animate()) accepts the same list of moments and a duration, for example “move this over 0.6 seconds.”
Example
@keyframes enter { from { opacity: 0 } to { opacity: 1 } }