ETYM Latin aggressio, from aggredi: cf. French agression.
In politics, an unprovoked attack often involving an escalating series of threats aimed at intimidating an opponent. The actions of Nazi Germany, under Adolf Hitler in the 1930s, leading to World War II were considered to be aggressive. The invasion of Kuwait by Iraq in 1990 was condemned as an act of aggression.
1. A disposition to behave aggressively.
2. A feeling of hostility that arouses thoughts of attack; SYN. aggressiveness.
3. Deliberately unfriendly behavior.
4. The act of initiating hostilities.
5. Violent action that is hostile and usually unprovoked; SYN. hostility.

(biology) In biology, behavior used to intimidate or injure another organism (of the same or of a different species), usually for the purposes of gaining a territory, a mate, or food. Aggression often involves an escalating series of threats aimed at intimidating an opponent without having to engage in potentially dangerous physical contact. Aggressive signals include roaring by wapiti (American Elk), snarling by dogs, the fluffing up of feathers by birds, and the raising of fins by some species of fish.