word for eternity
In the game of go, a ko occurs when one player could immediately recapture a stone that was just captured, and the opposing player could immediately recapture that stone, and so on, forever.
There is a rule in place to prevent this: No player may make a move that would result in a board position which has already occurred. This means that in a ko situation, the player whose piece was captured must play elsewhere before they can recapture. So, in the above picture, white (the "O's") must play elsewhere. If white's move is important enough, black may choose to respond to that move, rather than playing in the * position to "fill in." Often a ko will move back and forth over the course of a game this way.