[1] : 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

[2] : WordNet (r) 2.0

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

[3] : 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

Spark Plug >> S Words
Spark Plug, definition of term: Spark Plug
spark+plug_pag1.html

Plug >> P Words
Plug, definition of term: Plug
plug_pag1.html

Spark >> S Words
Spark, definition of term: Spark
spark_pag1.html

Plug Hat >> P Words
Plug Hat, definition of term: Plug Hat
plug+hat_pag1.html

Plug In >> P Words
Plug In, definition of term: Plug In
plug+in_pag1.html

Plug And Play >> P Words
Plug And Play, definition of term: Plug And Play
plug+and+play_pag1.html