Spark plug :  (noun)

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: sparking plug, 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,