Actually, there are two definitions of Octane, both rather interconnected. In one definition, Octane is a “a colorless flammable hydrocarbon of the alkane series, obtained in petroleum refining.” By refining fuel to enhance these hydrocarbons, refineries are in essence increasing octane in end use gasoline. Here, octane is actually a physical thing - a liquid.
As a measure of fuel quality, octane “is the measure of how much compression a fuel can withstand before igniting.” Or, in layman’s terms, the higher the octane rating, the less likely the fuel is going to pre-ignite at higher pressures and damage an engine. That’s why performance cars with higher compression engines require higher octane (premium) fuel. Here, octane is describes a type of fuel.
Engine “knock” is created by pre-ignition, or ignition of the fuel before the spark from the spark plug ignites it. This pre-ignition causes a pressure wave that rattles the piston, which creates the knock sound.
Octane ratings, or numbers at the gas pump, indicate the percentage/ratio of isooctane (the type of physical octane used in fuel) to heptane. A 92 octane fuel thus has 92% isooctane to 8% heptane. Straight-run gasoline typically has an octane rating of about 70 which ultimately requires refining to increase octane.
Though typical octane offerings at gas stations range from 85.5 to 91 octane, some stations offer fuels up to 101 octane. Gasoline with a rating higher than 100 typically contains other types of octane (or boosters) besides isooctane like alcohol or methane.