Aggression between groups is determined partly by willingness to fight, which depends on a number of factors including numerical advantage, distance from home territories, how often the groups encounter each other, competitive abilities, differences in body size, and whose territory is being invaded. Also, an individual is more likely to become aggressive if other aggressive group members are nearby. One particular phenomenon, the formation of coordinated coalitions, that raid neighboring territories to kill has only been documented in two species in the animal kingdom: common chimpanzees and humans; however, a number of classifications and dimensions of aggression have been suggested. These depend on such things as whether the aggression is verbal or physical; whether or not it involves relational aggression such as covert bullying and social manipulation; whether harm to others is intended or not; whether it is carried out actively or expressed passively; and whether the aggression is aimed directly or indirectly. Classification may also encompass aggression-related emotions (e.g. anger) and mental states (e.g. impulsivity, hostility).  Aggression may occur in response to non-social as well as social factors, and can have a close relationship with stress coping style.  Aggression may be displayed in order to intimidate.

The operative definition of aggression may be affected by moral or political views. An example of this is the axiomatic moral view called the non-aggression principle and the political rules governing the behavior of one country toward another. Likewise in competitive sports, or in the workplace, some forms of aggression may be sanctioned and others.