Reactive Action
Very similar to the Interruptive Action, the Reactive Action is taken in response to a specific trigger. However, a Reactive Action occurs immediately following the completion of the triggering action (instead of interrupting the action). Reactive actions cannot prevent or change the effects of the triggering action.
You are limited to taking only one Reactive or Interruptive Action per round. You may only use a single action in response to a triggering event (i.e. if you had two powers with a similar trigger, you would have to choose which power to utilize, even if they were different action types). You may not use Interruptive or Reactive actions during your turn.
Example
Dirkus makes an attack on his opponent, but misses. Not to be denied, he uses the Backswing ability as a Reactive Action, allowing him to make a Melee Basic Attack when he misses all targets of his attack.