The Collaborative International Dictionary of English v.0.48:

Aggressor \Ag*gres"sor\, n. [L.: cf. F. agresseur.]
   The person who first attacks or makes an aggression; he who
   begins hostility or a quarrel; an assailant.
   [1913 Webster]

         The insolence of the aggressor is usually proportioned
         to the tameness of the sufferer.         --Ames.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aggressor
    n 1: someone who attacks [syn: attacker, aggressor,
         assailant, assaulter]
    2: a confident assertive person who acts as instigator

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AGGRESSOR, crim. law. He who begins, a quarrel or dispute, either by
threatening or striking another. No man may strike another because he has
threatened, or in consequence of the use of any words.