Definition
Avgas is a high-octane gasoline fuel formulated specifically for piston-engine aircraft. It is distinct from automotive gasoline and from jet fuel (kerosene-based turbine fuel), and the most common grade in general aviation is 100LL (100 octane, low lead), dyed blue for identification.
Plain English
Avgas is the special gasoline used in piston-powered airplanes. It is not the same as car gas or jet fuel, and the standard type in light aircraft is a blue-dyed fuel called 100LL.
Context Anchor
Seen during fueling, preflight checks, fuel system discussions, and in warnings about fluid spills on composite aircraft parts.
Derivation
Avgas is a contraction of 'aviation gasoline.' The term came into use to clearly distinguish piston-engine aircraft fuel from jet fuel and from automotive gasoline, since mixing them up has serious consequences.
Why Pilots Care
Using automotive gasoline instead of avgas can destroy an engine in flight.
Intuition Check
Do not read gas here as any vapor or as jet fuel. In avgas, gas means liquid gasoline made for piston aircraft engines.
Example Sentence 1
Before departure, the pilot confirmed the truck had delivered 100LL avgas and checked the fuel sample for the correct blue color.
Example Sentence 2
Any avgas spilled on the composite wing was wiped off at once to avoid surface damage.