The Collaborative International Dictionary of English v.0.48:

Backlash \Back"lash`\ (b[a^]k"l[a^]sh`), n. [Back, adv. + lash.]
   1. (Mech.) The distance through which one part of connected
      machinery, as a wheel, piston, or screw, can be moved
      without moving the connected parts, resulting from
      looseness in fitting or from wear; also, the jarring or
      reflex motion caused in badly fitting machinery by
      irregularities in velocity or a reverse of motion.
      [1913 Webster]

   2. A strong and sudden reverse movement in a moving part of a
      machine.
      [PJC]

   3. A strong popular reaction serving to counter the effect of
      an action; -- used especially of adverse reactions to
      social or political developments.
      [PJC]

WordNet (r) 3.0 (2006):

backlash
    n 1: a movement back from an impact [syn: recoil,
         repercussion, rebound, backlash]
    2: an adverse reaction to some political or social occurrence;
       "there was a backlash of intolerance"
    v 1: come back to the originator of an action with an undesired
         effect; "Your comments may backfire and cause you a lot of
         trouble" [syn: backfire, backlash, recoil]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "backlash":
   antagonism, antipathy, backfire, backlashing, backwash, boomerang,
   bounce, bounce back, bound, carom, clashing, clout, collision,
   conflict, confutation, contradiction, contraposition, contrariety,
   contrecoup, counteraction, counterposition, counterworking,
   crankiness, crotchetiness, dissent, force, friction, impact,
   impress, impression, imprint, interference, kick, kick back,
   kickback, mark, nonconformity, opposition, opposure, oppugnance,
   oppugnancy, perverseness, print, reaction, rebound, rebuff,
   recalcitrance, recalcitration, recoil, reflex, renitency,
   repercussion, repugnance, repulse, resilience, resistance,
   response, revolt, ricochet, spring, swimming upstream