The Collaborative International Dictionary of English v.0.48:

Spark plug \Spark plug\
   In internal-combustion engines with electric ignition, a
   plug, screwed into the cylinder head, connected with the
   induction coil or magneto circuit on the outside, and used to
   produce a spark to ignite the fuel-air mixture in the engine.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

spark plug
    n 1: electrical device that fits into the cylinder head of an
         internal-combustion engine and ignites the gas by means of
         an electric spark [syn: spark plug, sparking plug,
         plug]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "spark plug":
   abettor, activator, actuator, animator, arouser, cajoler, coax,
   coaxer, encourager, energizer, firer, gadfly, galvanizer,
   human dynamo, impeller, inducer, inspirer, life, motivating force,
   motive power, mover, moving spirit, persuader, pleader,
   prime mover, prompter, restorative, spark, sparker, stimulant,
   stimulator, stimulus, tempter, tonic, wheedler