The Collaborative International Dictionary of English v.0.48:

Warder \Ward"er\, n.
   1. One who wards or keeps; a keeper; a guard. "The warders of
      the gate." --Dryden.
      [1913 Webster]

   2. A truncheon or staff carried by a king or a commander in
      chief, and used in signaling his will.
      [1913 Webster]

            When, lo! the king suddenly changed his mind,
            Casts down his warder to arrest them there.
                                                  --Daniel.
      [1913 Webster]

            Wafting his warder thrice about his head,
            He cast it up with his auspicious hand,
            Which was the signal, through the English spread,
            This they should charge.              --Drayton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

warder
    n 1: a person who works in a prison and is in charge of
         prisoners

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "warder":
   advance guard, air warden, armed guard, attendant, bank guard,
   bull, caretaker, castellan, coast guard, commandant, conservator,
   cordon, cordon sanitaire, curator, custodian, forest ranger,
   game warden, gamekeeper, gaoler, garrison, goalie, goalkeeper,
   goaltender, governor, guard, guarder, guardian, guardian angel,
   guardsman, inlying picket, jailer, janitor, keeper, lifeguard,
   lifesaver, next friend, outguard, outpost, picket,
   principal keeper, prison guard, prochein ami, ranger, rear guard,
   screw, security guard, shepherd, steward, train guard, turnkey,
   van, vanguard, warden