In a spark-ignition gasoline engine, a phenomenon that occurs when unburned fuel-air mixture explodes in the combustion chamber before being ignited by the spark. The resulting shock waves produce a metallic knocking sound. Loss of power occurs, which can be prevented by reducing the compression ratio, re-designing the geometry of the combustion chamber, or increasing the octane number of the gasoline (usually by the use of tetraethyl lead antiknock additives).

1. A beating; a rap; a series of raps.
2. The noise produced by beating.