The Collaborative International Dictionary of English v.0.48:

Aggressive \Ag*gres"sive\, a. [Cf. F. agressif.]
   1. Tending or disposed to aggress; having or showing
      determination and energetic pursuit of one's own ends at
      the expense of others or mindless of others' needs or
      desires; characterized by aggression; making assaults;
      unjustly attacking; as, an aggressive policy, war, person,
      nation; an aggressive businessman; an aggressive
      basketball player; he was aggressive and imperious in his
      convictions; aggressive drivers. Opposite of
      unaggressive.
      [1913 Webster + WordNet 1.5]

            No aggressive movement was made.      --Macaulay.
      [1913 Webster]

   2. Marked by self-confident ambition, vigorous
      competitiveness, energy and initiative; as, an aggressive
      young executive.

   Syn: enterprising, pushful, pushing, pushy
        [WordNet 1.5]

   3. (Med., Biol.) Tending to grow or spread quickly; as, an
      aggressive tumor. [Narrower terms: invasive (vs.
      noninvasive) ] --AS

   Syn: fast-growing(prenominal)
        [WordNet 1.5]

   4. Tending to initiate unprovoked attacks; initiating
      unprovoked military action; eager to fight; as, aggressive
      acts against another country.

   Syn: belligerent.
        [WordNet 1.5]

   Note: Narrower related terms: {bellicose, combative,
         pugnacious, scrappy, truculent ; {hard-hitting,
         high-pressure ; {hostile (used of attempts to buy or
         take control of a business: "hostile takeover";
         "hostile tender offer"); predatory, rapacious,
         raptorial, ravening, vulturine, vulturous . See also:
         assertive, hostile, offensive.
         [WordNet 1.5] -- {Ag*gres"sive*ly, adv. --
         Ag*gres"sive*ness, n.
         [1913 Webster]

WordNet (r) 3.0 (2006):

aggressive
    adj 1: having or showing determination and energetic pursuit of
           your ends; "an aggressive businessman"; "an aggressive
           basketball player"; "he was aggressive and imperious;
           positive in his convictions"; "aggressive drivers" [ant:
           nonaggressive, unaggressive]
    2: tending to spread quickly; "an aggressive tumor" [syn:
       aggressive, fast-growing(a), strong-growing]
    3: characteristic of an enemy or one eager to fight; "aggressive
       acts against another country"; "a belligerent tone" [syn:
       aggressive, belligerent]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "aggressive":
   abrupt, active, acute, adventuresome, adventurous, ambitious,
   animated, antagonistic, assertive, battling, bearish, beastly,
   bellicose, belligerent, bickering, bloodthirsty, bloody,
   bloody-minded, bluff, blunt, bold, brash, brisk, brusque, cavalier,
   chauvinist, chauvinistic, churlish, combative, contentious, crusty,
   curt, disputatious, divisive, domineering, driving, dynamic, enemy,
   energetic, enterprising, enthusiastic, eristic, factional,
   factious, ferocious, fierce, fighting, forceful, forcible, forward,
   full of fight, full of pep, go-ahead, go-go, gruff, hard-hitting,
   harsh, hawkish, hearty, hostile, hustling, imperious, impetuous,
   incisive, inimical, intense, irascible, irritable, jingo, jingoish,
   jingoist, jingoistic, keen, kinetic, litigious, lively, living,
   lusty, martial, masterful, mettlesome, militant, militaristic,
   military, offensive, on the offensive, partisan, peppy, polarizing,
   polemic, pugnacious, pushful, pushing, pushy, quarrelsome, robust,
   rough, saber-rattling, sanguinary, sanguineous, savage, scrappy,
   self-assertive, severe, sharp, short, shrewish, smacking, snappy,
   snippy, soldierlike, soldierly, spanking, spirited, strenuous,
   strong, surly, take-charge, take-over, tough, trenchant,
   trigger-happy, truculent, unfriendly, unpacific, unpeaceable,
   unpeaceful, up-and-coming, venturesome, venturous, vibrant,
   vigorous, vivacious, vivid, warlike, warmongering, warring,
   wrangling, zestful, zesty, zippy