Plain English Meaning
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.
Full 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.