It is the unit or scale which is used to measure the quality of fuel on the basis of anti-knocking property of internal combustion engine. It is defined as the percentage of iso-octane by volume in the mixture of iso-octane and n-heptane which has same anti-knocking property.
n-heptane è CH3-CH2-CH2-CH2-CH2-CH2-CH2
low anti knocking property
iso-octane è CH3-C-CH2-C-CH3
High anti-knocking property