Plain English Meaning
A type of high-octane piston-engine aircraft fuel that contains lead and is colored green so pilots and fuelers can tell it apart from other fuels at a glance.
Full Definition
A grade of leaded aviation gasoline with a minimum aviation lean rating of 100 and a minimum aviation rich rating of 130, used in higher-performance piston aircraft engines that require a high-octane fuel to resist detonation. AVGAS 100 is dyed green to distinguish it from other fuel grades.
Context Anchor
Seen in fuel-grade discussions, aircraft manuals, fuel placards, and fueling decisions for piston-engine aircraft.
Derivation
AVGAS is a contraction of 'aviation gasoline.' The number 100 refers to the fuel's octane-style performance rating — the higher the number, the better the fuel resists premature (detonation) under . Knowing this helps because the grade number directly tells you how much engine the fuel can handle before detonating.