The terms preignition and postignition can be used to indicate when in the engine cycle the abnormal ignition occurs, as described below.
Similarly, the term postignition refers to abnormal ignition of the fuel-air-residual gas mixture after the normal ignition (but that still resulting in a propagating flame).
It should be emphasized that the terms preignition and postignition refer to when the ignition (that is, the initiation of a propagating flame) occurs, and do not refer to the subsequent overall rate of combustion or combustion phasing.
The source of preignition (or postignition) can either be (1) some permanent combustion chamber component such as a hot spark plug electrode or a hot exhaust valve, or (2) some object derived from the fuel and/or lubricant that is present in the combustion chamber (historically this object has been thought to always be a combustion chamber deposit).
To avoid ambiguity, and be more clear about the source of the abnormal ignition, in this paper the following terms are used to refer to the types of preignition or postignition as classified by their source: