The stages in many 3D games (and at least one 2D game) have a “ring” as a fighting location, surrounded by walls that fence off the fighters, edges that do not, or both. For some games, the ring is an endless field.
For other games, the ring is often used to obtain a “ring out”; by forcing an opponent to leave the confines of the ring (or, easier but far less common, tricking them into landing themselves there, usually by bait-and-switch tactics), one can win a round without performing a knockout. These games and series allow ring-outs: