A spark plug is an electrical device that fits into the cylinder head of internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated wire which is connected with an 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.

Spark plugs are typically designed to have a variable spark gap. The spark gap may be adjusted by the technician installing the spark plug. A spark plug gap gauge is used to set the gap. The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly.

Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use glow plugs to improve cold start characteristics.

Sectional view of a typical spark plug