Definition of Octane Number

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.
Octane 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.
Octane numbers are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock).