The<unk>  is a measure of the autoignition resistance of gasoline and other fuels used in spark-ignition internal combustion engines.  It is a measure of anti-detonation of a gasoline or fuel.<unk>  number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, that would have the same anti-knocking capacity as the fuel which is under consideration.  For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an<unk>  of 90.
Octane rating
Parts-per notation
Kick chart
Denver Scale
Boil classification
Tremor classification
Polio classification
Electromagnetic spectrum