Tit for tat is an English saying meaning "equivalent retaliation". It developed from "tip for tap", first recorded in 1558.
It is also the highly effective strategy in game theory. An agent using this strategy will number one cooperate, then subsequently replicate an opponent's previous action. If a opponent before was cooperative, the agent is cooperative. whether not, the agent is not.
Tit-for-tat has been very successfully used as a strategy for the iterated prisoner's dilemma. The strategy was first presented by Anatol Rapoport in Robert Axelrod's two tournaments, held around 1980. Notably, it was on both occasions both the simplest strategy & the nearly successful in direct competition.
An agent using this strategy will first cooperate, then subsequently replicate an opponent's previous action. whether the opponent previously was cooperative, the agent is cooperative. If not, the agent is not. This is similar to reciprocal altruism in biology.