State-based action

State-based actions are game actions that happen when any of a set of improper conditions arise in the game.

Description
Whenever a player would gain priority, state-based actions first check for certain game conditions. If any of those conditions are met, the necessary actions are carried out simultaneously as a single event. Then, any triggered abilities go on the stack and the check is repeated. Once no conditions for state-based actions exist, the appropriate player gains priority.

A creature being destroyed by lethal damage, the "legend rule" and "world rule" are all state-based actions.

The silver-bordered card Rules Lawyer allows a player to ignore the effects of state-based actions.

Rules
In addition to the above, the Magic Tournament Rules include another state-based action for use in Sudden Death games: