1) Equalizing phrase 2) Even swap 3) Mechanical trade 4) One way to retaliate 5) Quid pro quo 6) Repayment in kind 7) Retaliation in kind 8) Retaliatory equivalent 9) Retributively 
Tit for tat is an English saying meaning `equivalent retaliation`. It is also a highly effective strategy in game theory for the iterated prisoner`s dilemma. The strategy was first introduced by Anatol Rapoport in Robert Axelrod`s two tournaments, held around 1980. Notably, it was (on both occasions) both the simplest strategy and the most suc...
A strategy in a repeated game (or a series of similar games). When a Prisoner's dilemma game is repeated between the same players, the tit-for-tat strategy is to choose the 'cooperate' action unless in the previous round, one's opponent chose to defect, in which case one responds by choosing to defect this round. This tends to induce cooperative be...
An effective strategy for playing the Iterated Prisoner's Dilemma. Tit-for-Tat starts by cooperating, and then does whatever its opponent did in the previous round of play. 
Tit for tat is London Cockney rhyming slang for a hat.
[n] - an equivalent given in return
noun an equivalent given in return