The Collaborative International Dictionary of English v.0.48:

Equalizer \E"qual*i`zer\, n.
   1. One who, or that which, equalizes anything.
      [1913 Webster]

   2. same as Equalizing bar.
      [Webster 1913 Suppl.]

   3. A device, as a bar, for operating two brakes, esp. a pair
      of hub brakes for an automobile, with equal force.
      [Webster 1913 Suppl.]

   4. (Elec.) Any device for equalizing the pull of
      electromagnets; also, a conductor of low resistance
      joining the armature ends of the series field coils of
      dynamos connected in parallel.
      [Webster 1913 Suppl.]

   5. (A["e]ronautics) A sliding panel to preserve the lateral
      stability of an a["e]roplane.
      [Webster 1913 Suppl.]

   6. (Electronics) a device or circuit within a
      sound-reproducing system that reproduces the original
      frequency distribution of the sound before recording by
      compensating for inequialities in the frequency response
      of the system.
      [PJC]

   7. a gun. [slang]
      [PJC]

WordNet (r) 3.0 (2006):

equalizer
    n 1: electronic equipment that reduces frequency distortion
         [syn: equalizer, equaliser]
    2: a weight that balances another weight [syn: counterweight,
       counterbalance, counterpoise, balance, equalizer,
       equaliser]
    3: a score that makes the match even

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "equalizer":
   KO, clincher, crusher, death stroke, deathblow, end-all, ender,
   final stroke, finisher, finishing stroke, kayo, kayo punch,
   knockout, knockout blow, last dab, quietus, settler, sockdolager,
   stopper