The Collaborative International Dictionary of English v.0.48:

catalyst \catalyst\ n.
   1. (Chem.) a substance that initiates or accelerates a
      chemical reaction without itself being affected; as,
      thousands of enzymes serve in concert as calaysts to
      produce the sequence of reactions we call "life"; the
      industrial production of cheap ammonia depended on finding
      a good catalyst.
      [WordNet 1.5]

   2. something that serves as a precipitating occasion for an
      event; as, the invasion acted as a catalyst to unite the
      country.

   Note: A catalyst is never the main cause of an event, but may
         serve to hasten events for which the underlying causes
         are present prior to the appearance or occurrence of
         the catalyst.
         [WordNet 1.5 +PJC]

   3. something or someone that causes events to happen with
      itself being changed.
      [PJC]

WordNet (r) 3.0 (2006):

catalyst
    n 1: (chemistry) a substance that initiates or accelerates a
         chemical reaction without itself being affected [syn:
         catalyst, accelerator] [ant: anticatalyst]
    2: something that causes an important event to happen; "the
       invasion acted as a catalyst to unite the country"

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "catalyst":
   agent, agent provocateur, agitator, agitprop, alterant, alterative,
   alterer, author, begetter, beginner, catalysis, catalytic agent,
   causer, creator, decay, demagogue, dialysis, dissociation,
   effector, engenderer, exciter, father, ferment, firebrand, fission,
   fomenter, generator, goad, hydrolysis, hydrolyst, impetus, impulse,
   incendiary, incentive, incitation, incitement, inciter, inflamer,
   innovationist, innovator, inspirer, instigator, introducer, leaven,
   maker, mischief-maker, modificator, modifier, mother, motivation,
   mover, originator, parent, photolysis, precursor, prime mover,
   primum mobile, producer, provocateur, provoker, rabble-rouser,
   ringleader, rouser, seditionary, seditionist, sire, splitting,
   spur, stimulant, thermolysis, transformer, transmogrifier,
   troublemaker, urger, yeast