Plain English Meaning
A metallic knocking or rattling noise from the engine that means the fuel is exploding inside the cylinders instead of burning smoothly. It is a warning sign that something is wrong with how the engine is running.
Full Definition
An audible metallic knocking sound produced inside a piston engine's cylinders when the fuel-air mixture ignites uncontrollably ahead of the spark plug's normal flame front, rather than burning smoothly. It is a symptom of detonation and indicates abnormal combustion that can damage the engine.
Context Anchor
Seen in fuel and oil discussions, especially when learning why the correct fuel grade matters for piston aircraft engines.
Derivation
From the sound itself -- a sharp metallic 'ping' or 'pinging' noise. The word imitates what or actually hears when combustion strikes the inside of the .