Aggression is a response to frustration by attacking either the source of frustration or a substitute.
Aggression is a primary drive (rather than the outcome of conflicting drives) that is not grounded in goal attainment. Rather its sole purpose is to release tension by means of a kind of breaking-up or a kind of destruction.
Aggression is a basic drive which natural selection has long favored so we have a large capacity for aggression, inherited from our ancestors, but lack opportunities to work it off in the modern world.
Aggression is a biologically rooted instinctive urge that promotes preservation of the self. If not expressed, aggressive energy accumulates and exerts increasing pressure on the organism for satisfaction.
Aggression is a nonhostile, nondestructive, constructive energy at the service of development, differentiation, mastery, object relations, and the maintenance of a satisfactory balance between the self and the environment.
Aggression is accompanied by intense erotic pleasure... it breaks down the boundaries between self and other, giving a voice to the need to possess and control.
Aggression is a need to overcome opposition forcefully, to fight, to revenge an injury, to attack or kill another, to punish another.