spark plug

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device inserted in the head of an internal-combustion engine cylinder that ignites the fuel mixture by means of an electric spark.
• noun Informal One who gives life or energy to an endeavor.

from The Century Dictionary.

• noun In internal-combustion motors, the apparatus which carries into the combustion-chamber the electric terminals, properly insulated, by which the charge of carbureted air or gas is electrically ignited at the proper time.

from the GNU version of the Collaborative International Dictionary of English.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The part of an internal combustion engine which forms a high-voltage electric spark which ignites the fuel-air mixture to begin the power stroke.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun electrical device that fits into the cylinder head of an internal-combustion engine and ignites the gas by means of an electric spark