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

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "aggression":
   adventuresomeness, adventurousness, aggravated assault,
   aggressiveness, ambitiousness, amphibious attack, antagonism,
   armed assault, assailing, assailment, assault, attack,
   banzai attack, bellicism, bellicosity, belligerence, belligerency,
   blitz, blitzkrieg, breakthrough, charge, chauvinism, combativeness,
   contentiousness, counterattack, counteroffensive, coup de main,
   crippling attack, dead set at, descent on, diversion,
   diversionary attack, drive, dynamism, encroachment, enterprise,
   enterprisingness, ferocity, fierceness, fight, flank attack, force,
   forcefulness, frontal attack, gas attack, get-up-and-get,
   get-up-and-go, getup, go, go-ahead, go-getting, go-to-itiveness,
   gumption, head-on attack, hostility, hustle, incursion,
   infiltration, initiative, inroad, invasion, irruption, jingoism,
   lightning attack, lightning war, martialism, mass attack,
   megadeath, militancy, militarism, mugging, offense, offensive,
   onset, onslaught, overkill, panzer warfare, pugnaciousness,
   pugnacity, push, pushfulness, pushiness, pushingness,
   quarrelsomeness, raid, run against, run at, rush, saber rattling,
   sally, shock tactics, sortie, spirit, spunk, strike, truculence,
   unfriendliness, unpeacefulness, unprovoked assault,
   up-and-comingness, venturesomeness, venturousness, warmongering,
   warpath