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]