Motion vectors
A per-pixel buffer recording how far each pixel moved since the previous frame, expressed in screen space.
Each pixel carries a small 2D vector: the difference between its position this frame and last, derived from current and previous clip-space coordinates. The buffer — often a channel of the G-buffer — feeds temporal reprojection, temporal upscalers and motion blur. Static surfaces move only with the camera, while moving and skinned objects need their previous transforms and bone matrices for the vector to be right.
The same idea underpins video compression since MPEG in the early 1990s, where motion vectors describe how blocks shift between frames instead of being re-encoded — the difference is only that the codec guesses the motion while the renderer knows it exactly.