An immediate action is a type of triggered action that lets you act outside of your turn. There are two kinds of immediate actions— interrupts and reactions—which determine the timing of the action relative to the resolution of the trigger.
Once Per Round: You can use only one immediate action per round.
Not On Your Turn: You can’t take an immediate action during your own turn.
Resolution: It is possible that an immediate action can itself trigger one or more other triggered actions. When this happens, refer to the rules for Multiple Trigger Resolution.
A Reaction allows you to act once the trigger has resolved. The triggering action, event, or condition occurs and is completely resolved before you take your reaction, except that you can interrupt a creature’s movement. If a creature triggers your immediate reaction while moving (by coming into range, for example), you take your action before the creature finishes moving but after it has moved at least 1 square.
An Interrupt lets you act before the triggering action, event, or condition occurs. If your action would prevent the triggering action from resolving (for instance, by moving out of range of a triggering attack), then the triggering action is lost.