Motion blur
The streak that appears when something moves during the camera's exposure — recreated so motion reads as smooth rather than strobing.
A still-rendered frame captures an infinitely short instant, so fast motion jumps in jerky steps between frames. A real camera gathers light over the whole exposure and smears whatever moves. Haeberli and Akeley showed in 1990 how an accumulation buffer could mimic this by blending several sub-frames together — correct but expensive.
Today's method is screen-based: a velocity buffer, often a channel in the G-buffer, stores how far each pixel has moved since the last frame, and the image is blurred along those vectors. Both the camera's own panning and individual object motion can be handled. Used in moderation — too much is nauseating — and closely related to depth of field, which smears across space instead.