According to the Robert A. Baron and Deborah R. Richardson book Human Aggression, aggression is defined as any form of behavior directed toward the goal of harming or injuring another living being who is motivated to avoid such treatment.
Aggression is a behavior, not an emotion. It may take the form of a physical and verbal act. It can even involve a refusal to act, depriving another person of a need and as a result harming the individual, according to the Walter Mischel, Yuichi Shoda and Ronald E. Smith book, Introduction to Personality.
Aggression is motivated. There is intent to hurt another person. Unintentional acts to harm another person is not considered aggression.
The target is a living being. The recipient must be motivated to avoid the aggressive treatment.
The individual punches a person, while the recipient ducks repeatedly to avoid getting hurt. This is considered an act of aggression. Mischel, Shoda and Smith clarify what is not considered aggression: An assisted suicide is not considered an act of aggression if the deceased wanted to die, despite it being considered manslaughter in the state of law.
Aggression can be found across cultures, age groups and gender. According to Wayne Weiten's book Psychology, evolutionary psychologists argue that aggressiveness is integral toward survival or reproductive advantage. Genes that promote aggressiveness are likely to be passed onto the next generation. Various theories exist onto why aggressiveness exists, but there is one definition accepted by that most psychologists.