Octane is a measure of how anti-knock a gasoline is.
Octane is a scale that goes from 0% to 100%.
For example, a fuel with 87 octane detonates more easily than one with 92 octane.
A 92 octane gasoline has a mixture of 92% iso-octane (C8H18) and 8% heptane (C7H16).
87 octane gasoline is 87% by volume iso-octane and 13% heptane.
Commercial gasolines are composed of a series of hydrocarbons and additives: antifoams, detergents, cleaners, octane boosters.
Iso-octane has an octane rating of 100% while heptane has 0%, by definition.