An attack or maneuver that occurs where an entity would have to theoretically let its guard down near a nearby attacker in order to concentrate on some delicate action.
Opportunity actions typically only occur when one entity is attempting a delicate task while within arms reach of an enemy. While the number of times this can be triggered per in game round or what can trigger it may vary from system to system, common examples are:
-Attempting to cast a spell while next to an enemy
-Attempting a ranged attack on an adjacent enemy
-Running wildly away from a group of enemies
-Actions that would otherwise distract the entity from being able to properly defend itself