What is a Grim Trigger Strategy in game theory and what is it used for?

A Grim Trigger Strategy is a type of strategy in game theory, particularly in repeated games, where a player cooperates with others until one player deviates from a cooperative agreement. Once a deviation occurs, the player using the Grim Trigger Strategy punishes the deviating player by reverting to a non-cooperative stance for the remainder of the game. This strategy is typically employed to sustain cooperation in situations where players have an ongoing relationship, as it discourages defection by threatening severe consequences. It is often used in contexts like economics, environmental agreements, and negotiations to promote long-term collaborative behavior.