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]