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How does the octane number of gasoline relate to engine knocking?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

The octane number is a measure of a fuel's ability to resist knocking within an internal-combustion engine.12

Knocking occurs when the air-fuel mixture in the cylinder undergoes premature, uncontrolled combustion, creating intense pressure waves that cause the engine to vibrate and potentially suffer damage. In a properly functioning engine, the fuel should burn smoothly from the point of ignition.34

The octane scale defines fuel quality by comparing a gasoline's performance to mixtures of two reference fuels: iso-octane, which resists knocking, and heptane, which knocks readily. For example, a fuel with an octane number of 90 performs identically to a mixture of 90 percent iso-octane and 10 percent heptane. Consequently, using gasoline with a higher octane number is the most common method to prevent knocking, especially in engines operating at high compression ratios.143