The Collaborative International Dictionary of English v.0.48:

Jailer \Jail"er\, n. [OE. jailer, gailer, OF. geolier, F.
   ge[^o]lier. See Jail.]
   The keeper of a jail or prison. [Written also jailor,
   gaoler.]

WordNet (r) 3.0 (2006):

jailer
    n 1: someone who guards prisoners [syn: prison guard,
         jailer, jailor, gaoler, screw, turnkey]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "jailer":
   advance guard, armed guard, bank guard, bull, coast guard,
   commandant, cordon, cordon sanitaire, custodian, gaoler, garrison,
   goalie, goalkeeper, goaltender, governor, guard, guarder, guardian,
   guardsman, inlying picket, keeper, outguard, outpost, penologist,
   picket, principal keeper, prison guard, rear guard, screw,
   security guard, train guard, turnkey, van, vanguard, warden,
   warder



Easton's 1897 Bible Dictionary:

Jailer
   (of Philippi), Acts 16:23. The conversion of the Roman jailer, a
   man belonging to a class "insensible as a rule and hardened by
   habit, and also disposed to despise the Jews, who were the
   bearers of the message of the gospel," is one of those cases
   which illustrate its universality and power.