Definition
A specialized gasoline formulated for use in piston-engine aircraft. AVGAS is produced to tighter quality standards than automotive gasoline, contains a controlled amount of tetraethyl lead to prevent engine knock (detonation), and is graded by octane rating. The most common grade in current use is 100LL (100 octane, low lead), which is dyed blue for identification.
Plain English
Aircraft-grade fuel made for piston engines. It is cleaner, more consistent, and burns differently than the gasoline used in cars, and it is colored so pilots can tell at a glance which type they are putting in the tank.
Context Anchor
You encounter AVGAS during preflight fuel checks, refueling, and fuel-and-oil discussions for piston-engine airplanes.
Derivation
A blend of 'aviation' and 'gasoline.' The shortened form became standard because pilots and ground crews needed a quick, unambiguous word to distinguish piston-aircraft fuel from jet fuel during refueling.
Why Pilots Care
Using the wrong fuel can cause engine failure or damage. AVGAS is required for safe operation of most piston aircraft.
Intuition Check
AVGAS does not mean any gasoline that happens to be used around airplanes. It means a specific aviation-approved fuel for piston-engine aircraft, and the required grade matters.
Example Sentence 1
After landing, the pilot taxied to the self-serve pump and added 20 gallons of 100LL AVGAS to each wing tank.
Example Sentence 2
Before flight, the student confirmed the fuel truck was delivering AVGAS and not jet fuel.