Definition
A grade of aviation gasoline (avgas) with an octane rating of 100 and a reduced lead content compared to earlier high-lead avgas formulations. It is dyed blue for visual identification and is the most widely available piston-aircraft fuel in the United States. It is approved for use in most reciprocating (piston) aircraft engines that require a 100-octane fuel.
Plain English
100LL is the blue-coloured petrol used by most small piston-engine aeroplanes. It has a high octane rating so the engine runs smoothly, and it still contains a small amount of lead -- less than older aviation fuels, but more than car petrol.
Context Anchor
You will see 100LL at airport fuel pumps, on fuel trucks, in airplane fuel requirements, and during preflight fueling checks.
Derivation
The '100' refers to the fuel's octane rating, which measures its resistance to detonation (premature combustion) inside the cylinder. 'LL' stands for 'Low Lead' -- low only in comparison to earlier 100/130 avgas, which contained roughly twice as much lead. Knowing this helps the pilot recognise that 100LL is defined by both its anti-knock performance and its lead content.
Why Pilots Care
Using the correct grade prevents detonation, engine damage, and excessive lead fouling of spark plugs and valves.
Intuition Check
Do not read 100LL as 100 liters or as unleaded fuel. It means 100-grade low-lead aviation gasoline, and it still contains lead.
Example Sentence 1
Before departure, the pilot confirmed the fuel truck was dispensing 100LL and checked that the sample drained from the sumps was the correct blue colour.
Example Sentence 2
Before the lesson the instructor verified the tanks contained 100LL matching the engine requirements in the operating handbook.