Aggression is defined by Dr Karen Overall in the textbook, Clinical Behavioral Medicine for Small Animals as
“an appropriate or inappropriate threat or challenge that is ultimately resolved by combat or deference.”
An aggressive display is a set of behaviors that are threatening or challenging, but aim at making the recipient of the challenge go away without provoking a fight.
Aggression can be broadly categorized into offensive and defensive types depending on the emotion underlying the behavior, and can also be described according to the suspected underlying reason. For example, dogs can show territorial aggression, predatory aggression, and resource guarding.
Some breeds have more of a tendency towards certain kinds of aggression than others, but a dog that shows aggression towards his owners, or indiscriminately towards strangers is usually considered to be behaviorally abnormal.