True misdirected aggression is the result of the combination of barrier frustration paired with any other form of aggression that is causing a high degree of mental arousal.
This may result in any nearby target, whether it be an object, animal, or person becoming a substitute outlet for the unreachable stimulus which originally caused the reaction.
Misdirected aggression can vary in how extreme the redirection is based on the dog's individual genetics. Some dogs may "snap out of it" before actually biting another target and some may totally commit to unleashing their full aggression on that surrogate.
The term is loosely used when people and other animals get "caught in the crossfire" of a confused dog in a fight, although these incidents are better classified as accidental bites.
Misdirected aggression is sometimes used incorrectly to label dominance aggression, such as when a dog consciously attacks another animal or person for interfering with their decision to become aggressive at another target. This can be explained through disciplinary behavior associated with leadership roles of a dominant dog.
It is possible for a dog to be prone to multiple reasons to act aggressively. As in all cases, multi-tiered aggression must be addressed by focusing on each type separately even if the presentation is similar. When a handler is unsure what the motivation for the aggression is, it is best to treat for both types.