The Collaborative International Dictionary of English v.0.48:

Setback \Set"back`\ (s[e^]t"b[a^]k`), n.
   1. (Arch.) Offset, n., 4.
      [1913 Webster]

   2. A backset; a countercurrent; an eddy. [U. S.]
      [1913 Webster]

   3. A reversal of progress in an endeavor; a reverse; a
      backset; a check; a repulse; a relapse. [Colloq. U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

setback
    n 1: an unfortunate happening that hinders or impedes; something
         that is thwarting or frustrating [syn: reverse,
         reversal, setback, blow, black eye]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "setback":
   arrest, arrestation, arrestment, atavism, backset, backsliding,
   backward deviation, backward motion, backward step, bafflement,
   balk, betrayed hope, blasted expectation, blighted hope, blockage,
   blocking, blow, buffet, check, checkmate, clogging, closing up,
   closure, comedown, confusion, constriction, cramp,
   cruel disappointment, dash, dashed hope, deceleration, defeat,
   delay, descent, detainment, detention, disappointment,
   discomfiture, disillusionment, dissatisfaction, down, drag,
   ease-off, ease-up, failure, fallen countenance, falling back,
   fiasco, fixation, fizzle, flagging, foil, foiling, foot-dragging,
   forlorn hope, frustration, hampering, hindering, hindrance,
   holdback, holdup, hope deferred, impediment, inhibition,
   interference, interruption, lag, lapse, let, letdown, letup,
   minus acceleration, mirage, negativism, nuisance value, obstacle,
   obstruction, obstructionism, occlusion, opposition, reaction,
   rebuff, recession, recidivation, recidivism, recrudescence,
   recurrence, reentry, refluence, reflux, regress, regression,
   relapse, renewal, repression, repulse, resistance, restraint,
   restriction, retardation, retardment, retroaction, retrocession,
   retroflexion, retrogradation, retrogression, retrusion, return,
   reversal, reverse, reverse of fortune, reversion, rollback, rout,
   severe check, slack-up, slackening, slowdown, slowing,
   slowing down, slowup, sore disappointment, squeeze, sternway,
   stranglehold, stricture, stumbling block, suppression,
   tantalization, tease, throwback