Plain English Meaning
A chemical you can add to fuel in tiny amounts to stop water in the fuel from freezing and clogging the fuel system when the aircraft is flying in very cold conditions.
Full Definition
A fuel system icing inhibitor additive, blended into aviation gasoline or jet fuel in small, specified concentrations to prevent water dissolved or suspended in the fuel from freezing into ice crystals that could block fuel filters or fuel lines at low temperatures.
Context Anchor
Seen in fuel-system icing and cold-weather prevention procedures, especially when discussing approved fuel additives.