Complicated subject, but Oldskool gave a nice short def as you asked. There is a lot of discussion around this subject and a lot of it on just the definitions of the different names of the event. Detonation, knock, pre ignition, surface ignition, pinging, pinking, spark knock, post ignition, etc. An event where the fuel air charge within the combustion chamber does not ignite at the appropriate time and/or does not burn in an orderly manner. During normal ignition the charge does not explode, once ignited by the spark plug the charge burns with the flame front moving across the combustion chamber in an orderly manner, until all the charge has been consumed. The time of the burn is as important as a proper burn. One usually aims to have the peak pressure at a certain degrees atdc for maximum work. In an engine with a proper charge ignition and burn rate the position of the peak pressure is controlled by the spark timing. Say 18 degrees btdc depending on the burn rate and the shape of the combustion chamber. If the combustion is not orderly with abnormal burning, the time of the peak pressure will not be controlled. Abnormal fuel air burn can be grouped into Pre-ignition or detonation. Detonation is the spontaneous combustion of the charge remaining in the chamber after the spark plug has ignited a normal burn. This spontaneous combustion will cause a sudden pressure increase. Detonation can be heard as pinging or knocking in the engine. Pre-ignition is the spontaneous ignition of the charge before the spark plug ignites. This causes a pressure build up before the piston has moved towards the top of the cylinder. Pre-ignition can cause serious damage quickly. Most of the time people refer all abnormal ignition burns as detonation. Not that it matters what you call it, detonation/pre-ignition is something we strive to avoid.
Some causes are: High compression ratio; lean mixture; fuel quality; timing; charge temp; incorrect spark plug; high engine operating temp; chamber design; carbon deposits; cracked plug; burned valve; high cylinder pressure; poor condition of internal components;etc.
Some results are: Overheating; bearing failure; burned valves; holes melted in piston tops; broken ring lands; seized rings; blow by; engine failure.
Too often builders chose a compression ratio then try to find fuel or adjust timing to run the engine, it's best to build the engine to suit the available fuel. If that requires a low compression ratio, so be it. One loses more power by backing off timing than lowering the CR.
