Definition
A rating that indicates a fuel's resistance to detonation (uncontrolled combustion) inside a piston engine. In aviation gasoline (avgas), the octane rating tells the pilot which fuel grade is approved for the engine; using a lower-rated fuel than specified can cause the fuel-air mixture to ignite prematurely or explode rather than burn smoothly, damaging the engine.
Plain English
A number on the fuel that tells you how well it resists exploding the wrong way inside the engine. Higher number means the fuel can handle more pressure and heat before it misbehaves.
Context Anchor
Pilots encounter octane when selecting or checking aviation gasoline for a piston-engine airplane.
Derivation
From the chemical compound 'octane' -- a hydrocarbon with eight carbon atoms (Greek 'okto' = eight). One specific form of octane resists premature ignition very well, so it was chosen as the reference standard for rating fuels. The number you see (e.g., 100) compares a fuel's anti-knock performance to that reference.
Why Pilots Care
Selecting fuel with the correct octane rating prevents detonation that can damage or destroy the engine.
Intuition Check
Octane does not mean the fuel has more power. It means the fuel is better at resisting harmful, too-sudden burning in the engine.
Example Sentence 1
The POH specifies 100LL avgas, so the pilot refused a lower-octane fuel offered at the unfamiliar field.
Example Sentence 2
Using fuel below the recommended octane rating risks detonation during a high-power climb.