Keyframe
A saved moment in an animation; the frames between keyframes are filled in for you.
See it in SVG path animator. Press Copy CSS and look for @keyframes: the start and end of the line drawing.
In plain words
Drawing every frame of an animation by hand takes a long time. Instead, you set a few important moments, such as where a shape starts and where it ends. Those moments are keyframes. The computer works out every frame between them. Easing decides how the in-between frames are spread out.