Source : Webster's Revised Unabridged Dictionary (1913)

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

   Syn: Attack; offense; intrusion; provocation.

Source : WordNet®

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