Uh...
anything which causes the A/F charge to ignite prematurely is detonation!
Too high a CR for the fuel used,
too high a charge temperature (intake temp or fuel temp), too advanced a spark, too low an octane fuel for the CR, too little engine cooling, the wrong heat range spark plug, carbon deposit hot spots in the cylinder, or the cumulative effect of any combination of the above.
And, it doesn't only affect forced aspirated (turbo, s/c) engines either, today's naturally aspirated engines are just as prone to it.
Modern engine management software will advance the spark to just shy of the detonation threshold to achieve maximum efficiency. As such, it doesn't take too much of an adverse condition to throw it beyond that threshold.
Once the knock sensor senses the increase in vibration, which it attributes to knock, it increases it's base signal to the ECU which then begins to retard the spark either to a pre-programmed level, or it will advance the spark until detonation is achieved and then retard it back slightly from that mark.
Because of this, naturally aspirated engines
are more sensitive than ever to such things as the intake temperature.
