Tweening
Generating the in-between frames between two keyframes automatically, so an animation needs only a start and an end state.
The word shortens "in-betweening", borrowed from hand-drawn animation where junior artists drew the frames between a lead animator's keyframes. In code, tweening interpolates a property — position, scale, colour, opacity — from a start value to an end value over a duration, computing each frame from the elapsed time.
The raw interpolation is linear; an easing function reshapes it into something that accelerates or overshoots. Flash popularised tweening for web animation in the 2000s, and the pattern lives on in every UI toolkit and game engine as the standard way to move something smoothly without hand-drawing every frame.