Aggression is behavior aimed at opposing or attacking something or someone. Though often done with the intent to cause harm, some might channel it into creative and practical outlets. It may occur either reactively or without provocation. In humans, aggression can be caused by various triggers. For example, built-up frustration due to blocked goals or perceived disrespect. Human aggression can be classified into direct and indirect aggression; while 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").

Dollard et al. (1939) proposed that aggression was due to frustration, which was described as an unpleasant emotion resulting from any interference with achieving a rewarding goal. Berkowitz extended this frustration–aggression hypothesis and proposed that it is not so much the frustration as the unpleasant emotion that evokes aggressive tendencies, and that all aversive events produce negative affect and thereby aggressive tendencies, as well as fear tendencies. Besides conditioned stimuli, Archer categorized aggression-evoking (as well as fear-evoking) stimuli into three groups; pain, novelty, and frustration, although he also described looming, which refers to an object rapidly moving towards the visual sensors of a subject, and can be categorized as "intensity."

Aggression can have adaptive benefits or negative effects. Aggressive behavior is an individual or collective social interaction that is a hostile behavior with the intention of inflicting damage or harm. Two broad categories of aggression are commonly distinguished. One includes affective (emotional) and hostile, reactive, or retaliatory aggression that is a response to provocation, and the other includes instrumental, goal-oriented or predatory, in which aggression is used as a means to achieve a goal. An example of hostile aggression would be a person who punches someone that insulted him or her. An instrumental form of aggression would be armed robbery. Research on violence from a range of disciplines lend some support to a distinction between affective and predatory aggression. However, some researchers question the usefulness of a hostile versus instrumental distinction in humans, despite its ubiquity in research, because most real-life cases involve mixed motives and interacting causes.

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. Examples are 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 not (see Workplace aggression). Aggressive behaviors are associated with adjustment problems and several psychopathological symptoms such as antisocial personality disorder, borderline personality disorder, and intermittent explosive disorder.