Definition of "spark plug"

spark plug

noun
plural spark plugs
1. 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.
  Quotations
  With all engines using spark plugs it is comparatively easy to ascertain whether the ignition system is working properly.
  1919 January, Practical hints on running a gas engine, United States Department of Agriculture
2. (figuratively) Someone who is a driving force in new endeavours.
  Quotations
  Steve Pincus, a Madison-area farmer who sells produce to L'Etoile, says admiringly of Odessa: "She's a spark plug. She's out there speaking out about sustainable agriculture and local food."
  2003, Growing for Market: News and Ideas for Market Gardeners
  Mr. Santiago used a sparkplug to smash the driver’s side window.
  2009 May 30, Al Baker, “Investigators Reviewing a Timeline of Seconds That Led to a Police Shooting”, in New York Times‎
3. (military) A rod (for cylindrical secondaries) or sphere (for spherical secondaries) of fissile material at the center of the secondary of a multistage thermonuclear weapon, which undergoes nuclear fission when compressed to supercriticality by the imploding secondary, helping to raise the secondary to the pressures and temperatures necessary to ignite nuclear fusion in the secondary's fusion fuel.