Much like a swift action, an immediate action consumes a very small amount of time but represents a larger expenditure of effort and energy than a free action. However, unlike a swift action, an immediate action can be performed at any time, even if it's not your turn.
Immediate actions tend to be somewhat rare, as they can be very powerful. Often, these are spells or class abilities which allow a character to do something special outside of their turn.
You may never take more than one immediate action per round.
You cannot use an immediate action if you are flat-footed.
As a note, immediate actions are different from attacks of opportunity. Making one or more attacks of opportunity does not use up your immediate action for the round.
Common uses for immediate actions include:
- Using a spell with an Immediate Casting time, such as Feather Fall, or Bungle
- Using a class or racial ability that allows immediate actions