The Collaborative International Dictionary of English v.0.48:

Aggression \Ag*gres"sion\, n. [L. aggressio, fr. aggredi: cf. F.
   agression.]
   The first attack, or act of hostility; the first act of
   injury, or first act leading to a war or a controversy;
   unprovoked attack; assault; as, a war of aggression.
   "Aggressions of power." --Hallam
   [1913 Webster]

   Syn: Attack; offense; intrusion; provocation.
        [1913 Webster]

WordNet (r) 3.0 (2006):

aggression
    n 1: a disposition to behave aggressively
    2: a feeling of hostility that arouses thoughts of attack [syn:
       aggression, aggressiveness]
    3: violent action that is hostile and usually unprovoked [syn:
       aggression, hostility]
    4: the act of initiating hostilities
    5: deliberately unfriendly behavior