The Collaborative International Dictionary of English v.0.48:

Fighter \Fight"er\ (f[imac]t"[~e]r), n. [AS. feohtere.]
   1. One who fights; a combatant; a warrior. --Shak.
      [1913 Webster]

   2. A boxer; a pugilist.
      [PJC]

   3. A person with the determination and will to persist
      through great difficulty to achieve a goal; one with the
      courage to fight and resist an opponent, and to struggle
      with all one's powers.
      [PJC]

   4. (Mil.) A military aircraft designed to seek out and
      destroy enemy aircraft; it is usually smaller and more
      maneuvarable than aircraft designed specifically for
      bombing. However, hybrid fighter-bomber aircraft that
      perform both functions also are used.
      [PJC]

WordNet (r) 3.0 (2006):

fighter
    n 1: someone who fights (or is fighting) [syn: combatant,
         battler, belligerent, fighter, scrapper]
    2: a high-speed military or naval airplane designed to destroy
       enemy aircraft in the air [syn: fighter, fighter
       aircraft, attack aircraft]
    3: someone who fights for a cause [syn: champion, fighter,
       hero, paladin]