Octane is an organic chemical, specifically an alkane, with formula C8H18. (8 carbon alkene). Octane has many isomers. One of those isomeres, the isooctane (2,2,4-trimetylpentane) is especially important, because its octane rating is by definition 100. So it is an important constituent of petrol (US:gasoline).
Octane ratings are ratings used to represent the anti-knock performance of petrol by giving the percentage of octane in an octane-heptane mixture that would have the same performance.