Tit-for-tat
Start by cooperating, then simply do whatever your partner did last time.
What it means
A strategy for the repeated prisoner's dilemma that opens with cooperation and thereafter mirrors the opponent's previous move, rewarding cooperation and punishing defection. In Axelrod's famous computer tournaments it triumphed despite its simplicity, owing to four properties: it is nice (never defects first), retaliatory (punishes defection), forgiving (returns to cooperation once the partner does), and clear (easy to read and reciprocate). Its success demonstrated that cooperation can emerge among self-interested agents through reciprocity, without central authority. It matters as a cornerstone of the theory of the evolution of cooperation, though it is vulnerable to error-driven feuds, which 'generous' variants soften.
Examples
In repeated play, you cooperate until your partner defects, defect once in response, then cooperate again the moment they do.
Flatmates who each wash up after the other did are running tit-for-tat: one skipped turn triggers one skipped turn back, and a single kind gesture restores the peace.
Two rival airlines hold their fares until one cuts; the other matches for exactly one season, then restores its price the moment the rival does — nice, retaliatory, forgiving, and easy to read.
First described in Anatol Rapoport; Axelrod (1984).