The Collaborative International Dictionary of English v.0.48:

Paladin \Pal"a*din\, n. [F., fr.It. paladino, fr. L. palatinus
   an officer of the palace. See Palatine.]
   A knight-errant; a distinguished champion; as, the paladins
   of Charlemagne. --Sir W. Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

paladin
    n 1: someone who fights for a cause [syn: champion, fighter,
         hero, paladin]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "paladin":
   Achilles, David, Hector, Roland, Samson, a man, advocate,
   apologist, brave, bulldog, champion, chutzpanik, decorated hero,
   defender, demigod, demigoddess, fighting cock, gallant, gamecock,
   good soldier, guard, hero, heroine, lion, man of courage,
   protector, stalwart, supporter, the brave, tiger, upholder,
   valiant, valiant knight, vindicator