Proper combustion in the chamber progresses from the sparkplug when it fires, across to the other side of the combustion chamber (of course this happens in miliseconds). The flame front moves across smoothly, and evenly. If some or all of the fuel ignites at once instead of progressively that's detonation. Also known as knock or ping. In a gasoline engine severe detonation eventually leads to damaged pistons, upper rings, and perhaps other components. In diesel engines it's the norm (no spark plug, just heat of compression for ignition source).

It is NOT pre-ignition as some mistakenly believe, nor is it run-on.