Aggression is overt or covert, often harmful, social interaction with the intention of inflicting damage or other harm upon another individual; although it can be channeled into creative and practical outlets for some. It may occur either reactively or without provocation. In humans, aggression can be caused by various triggers, from frustration due to blocked goals to feeling disrespected. Human aggression can be classified into direct and indirect aggression; whilst the former is characterized by physical or verbal behavior intended to cause harm to someone, the latter is characterized by behavior intended to harm the social relations of an individual or group.

In definitions commonly used in the social sciences and behavioral sciences, aggression is an action or response by an individual that delivers something unpleasant to another person. Some definitions include that the individual must intend to harm another person.

In an interdisciplinary perspective, aggression is regarded as "an ensemble of mechanism formed during the course of evolution in order to assert oneself, relatives or friends against others, to gain or to defend resources (ultimate causes) by harmful damaging means ... These mechanisms are often motivated by emotions like fear, frustration, anger, feelings of stress, dominance or pleasure (proximate causes) ... Sometimes aggressive behavior serves as a stress relief or a subjective feeling of power." Predatory or defensive behavior between members of different species may not be considered aggression in the same sense.

Aggression can take a variety of forms, which may be expressed physically, or communicated verbally or non-verbally: including anti-predator aggression, defensive aggression (fear-induced), predatory aggression, dominance aggression, inter-male aggression, resident-intruder aggression, maternal aggression, species-specific aggression, sex-related aggression, territorial aggression, isolation-induced aggression, irritable aggression, and brain-stimulation-induced aggression (hypothalamus). There are two subtypes of human aggression: (1) controlled-instrumental subtype (purposeful or goal-oriented); and (2) reactive-impulsive subtype (often elicits uncontrollable actions that are inappropriate or undesirable). Aggression differs from what is commonly called assertiveness, although the terms are often used interchangeably among laypeople (as in phrases such as "an aggressive salesperson").