Plain English Meaning
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.
Full 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.