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]