Aggression refers to any act that is intended to cause pain, suffering, or damage to another person.
It includes physical or verbal behavior intended to hurt someone. In laboratory experiments, this might mean delivering electric shocks or saying something likely to hurt another's feelings. Aggression is a basic drive of life and part of the process of survival and evolution.
Aggression in humans is partly genetic, with origins going as far as to our reptilian ancestors, and partly a result of the upbringing. Enhanced levels of aggression in male mice and monkeys have been associated with the hormone monoamine oxidase A, MAO-A. However, studies in macaque and humans showed that its negative effects can usually be compensated by parenting.
Other /More definition:
Aggression refers to the behavior that is intended to cause harm to another person.
Other /More definition:
Aggression refers to any action carried out with the intention of causing harm; hurting another person or achieving one's goals at the expense of another person. It is a behavior intended to physically or psychologically harm another person. In laboratory experiments, this might mean delivering electric shocks or saying something likely to hurt another's feelings. By this social psychological definition, one can be socially assertive without being aggressive.