Engine detonation is spontaneous combustion inside the cylinder AFTER the spark plug fires.
During normal ignition, the spark plug fires just before the piston reaches TDC. The flame travels across the combustion chamber igniting the Air/Fuel Mixture. This causes a steady increase in cylinder pressure and forces the piston down on the Power Stroke.
When detonation happens, some of the air/fuel ignites before the normal burn can get to it. This causes a brief, but intense, spike in cylinder pressure.
Detonation is also called an “Engine Knock”, “Knocking”, or “Pinging” because of the sound it makes.