the.com/keyframe

means A keyframe is a defined point in an animation or video that fixes a value — a position, shape, or setting — with everything between keyframes filled in automatically by interpolation.

from From traditional cel animation, where senior 'key animators' drew the critical poses of a motion — the extremes and turning points — while junior 'in-betweeners' filled in the rest frame by frame. 'Key' had long meant 'essential' or 'pivotal' (as in 'key point'), and 'frame' is simply a single image in a film reel. When computer animation and video compression adopted the workflow, the key drawing became the keyframe — the data point that matters — and the in-betweeners were replaced by software doing the same job, just faster and without the coffee breaks.