Aggression refers to behavior between members of the same species that is intended to cause humiliation, pain, or harm.

Ferguson and Beaver defined aggressive behavior as "Behavior which is intended to increase the social dominance of the organism relative to the dominance position of other organisms”

Aggression takes a variety of forms among humans and can be physical, mental, or verbal.

There are two types of aggression - hostile, affective aggression and instrumental, predatory, or goal-oriented aggression.

Reactive relational aggression (hostile, affective, retaliatory) is used in response to feeling attacked, threatened, or mad. Usually the person who shows this type of aggression feels provoked to do so. Instrumental relational aggression (predatory, goal-oriented) is used in order for an individual to get what they want.

Predatory or defensive behavior between members of different species is not normally considered as "aggression."

Like most, or even all behaviors, aggression can be examined in terms of its ability to help an animal reproduce and survive. Animals may use aggression to gain and secure territories, as well as other resources including food, water, and mating opportunities.

The most apparent type of aggression is that seen in the interaction between a predator and its prey. An animal defending itself against a predator becomes aggressive in order to survive and predator in order to secure food. Because aggression against a much larger enemy or group of enemies would be nearly certain to lead to the death of an animal, animals have developed a good sense of when they are outnumbered. This ability to gauge the strength of other animals gives animals a "fight or flight" response to predators; depending on how strong they gauge the predator to be, animals will either become aggressive or flee.

Although humans share aspects of aggression with non-human animals, they differ from most of them in the complexity of their aggression because of factors such as culture, morals, and social situations.

Aggression research 
Aggression is a behavior where one harms another individual intentionally. Over the years, there has been an ongoing debate on the origin or causes of aggression among humans. Some theories argue that aggression is innate, while others contend it is a learned behavior.

 The cognitive approach claims aggression is learned. The main argument of this theory is that people learn to be aggressive. However, Albert Bandura asserted that aggression is imitated rather than learned through conditioning. Apart from imitation, observation learning is another way people learn to be aggressive. For example, watching aggressive acts, especially in movies or video games, inc eases the likelihood of one acting aggressively. This mostly happens among children when children are exposed to aggressive environments. Children in such a situation usually grow, knowing that aggressive behavior is acceptable. Research has repeatedly depicted that children who are exposed to family violence as they grow up are more likely to develop aggressive acts or turn to aggressive adults in the future.
 The psychoanalytic approach views aggression as an innate. Sigmund Freud’s aggression theory describes aggressive behaviors as an innate drive or instinct, and it is not influenced by situations or nature.<ref> Thus, it is an inevitable part of human life.

References 

Human behavior