Reactive aggression is aggression that occurs in the face of a real and overt threat.
An example of this is a dog that seems perfectly friendly until someone makes a move that it interprets as threatening (think someone attacking its owner). It will then attack the person who made the "threatening" gesture. Reactive aggression is "justified" aggression. This is the opposite of proactive aggression which occurs when there is an aggressive reaction with little or no provocation.