Ko

In the game of Go, the rule of Ko (a term meaning eternity in Japanese) prevents repeating indefinitely between adjacent intersections, now the capture of a stone by a player, and then the capture of the capturing stone by the opponent.

Source: tatoeba (10370893)

Black plays E8 and captures the white stone on D8. On E8, the black stone is at Ko, that is, on the next move, White will be prohibited from capturing it by playing D8. White then plays E2, practically forcing Black to play E1. Now White can play D8 and capture the Ko stone.

Source: tatoeba (10371150)

In order to recapture the Ko, Black now forces White to play elsewhere on the board and, on the next move, repeats the previous attack. And the fight goes on like this, in what is called the battle of Ko.

Source: tatoeba (10371792)

In a battle of Ko, there are several moves of a strategic nature on practically the entire board.

Source: tatoeba (10371849)