Reference Fuel – In the measurement of octane quality of gasoline a mixture of two hydrocarbons called iso-octane and normal heptane. Iso-octane is assigned an octane rating of 100, and normal heptane is assigned an octane rating of zero. A blend of 90 per cent iso-octane and 10 per cent n-heptane would have an octane rating of 90 octane.

(R + M) / 2 – The average of Research Octane Number and Motor Octane Number; one measure of the combustion quality of gasoline.