Spark plug, the part of an internal-combustion engine that provides the spark to ignite the mixture of fuel and air. The most common type of spark plug is that used in automobile engines. It has two electrodes, separated by a gap.

The spark jumps across this gap when a very high voltage is applied from either a battery and ignition coil or from a magneto. One electrode is encased in an insulator, usually made of porcelain, that separates it from the steel base of the plug. The other electrode is attached directly to the base of the plug and is grounded to the engine block when the spark plug is screwed into its socket.

Spark plugs are also used in most jet and turboprop engines. The plugs are required to ignite the fuel to start these engines, but once combustion has started, the fuel burns continuously as it enters the combustion chamber.