Plain English Meaning

A small screw-in part that produces an electric spark to set fire to the fuel and air mix inside an engine cylinder. Without it, the engine has no way to start the burn that drives the piston down.

Full Definition

A removable component threaded into the cylinder head of a reciprocating engine that delivers a high-voltage electrical spark across a small air gap between two electrodes, igniting the compressed fuel-air mixture inside the combustion chamber. Aircraft spark plugs are typically shielded to prevent radio interference and use platinum or iridium electrodes for long under high and pressures.