- (of gasoline) a designation of antiknock quality, numerically equal to the percentage of isooctane by volume in a mixture of isooctane and normal heptane that matches the given gasoline in antiknock characteristics.
- a measure of the quality of a petrol expressed as the percentage of isooctane in a mixture of isooctane and n -heptane that gives a fuel with the same antiknock qualities as the given petrol
