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]