Source : Webster's Revised Unabridged Dictionary (1913)

Spark plug \Spark plug\
   In internal-combustion engines with electric ignition, a
   plug, screwed into the cylinder head, having through it an
   insulated wire which is connected with the induction coil or
   magneto circuit on the outside, and forms, with another
   terminal on the base of the plug, a spark gap inside the
   cylinder.

Source : WordNet®

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}]