Pronunciation: (u-gresh'un),[key] —n. 1. the action of a state in violating by force the rights of another state, particularly its territorial rights; an unprovoked offensive, attack, invasion, or the like: The army is prepared to stop any foreign aggression. 2. any offensive action, attack, or procedure; an inroad or encroachment: an aggression upon one's rights. 3. the practice of making assaults or attacks; offensive action in general.
4. Psychiatry.overt or suppressed hostility, either innate or resulting from continued frustration and directed outward or against oneself.