Plain English Meaning
The standard blue-dyed aviation gasoline used by most piston-engine aircraft. The 100 refers to its octane rating, and the LL means it has less lead in it than the older high-octane fuel it replaced.
Full Definition
A grade of leaded aviation gasoline with a minimum octane rating of 100, containing a reduced amount of tetraethyl lead compared to earlier high-octane aviation fuels. It is dyed blue for visual identification and is the most widely used piston-engine aviation fuel in general aviation today.
Context Anchor
You will see AVGAS 100LL on fuel trucks, airport fuel pumps, fuel caps, and in the fuel section of an aircraft flight manual.
Derivation
is simply a contraction of 'aviation gasoline.' The '100' is the octane rating — a measure of the fuel's to . 'LL' stands for ' Lead,' indicating it contains less tetraethyl lead than the older grade it replaced. Despite the name, still contains lead — it is just lower than its predecessor.