What does “residual fuel” refer to in aviation?

Residual fuel in aviation specifically refers to the fuel that remains in the tanks after the engine has been shut down. This term encompasses any fuel that has not been consumed during flight operations or that hasn't been drained or transferred from the fuel tank. Understanding residual fuel is important for various reasons, including ensuring accurate fuel management, weight calculations, and safety measures during refueling operations. Recognizing how much fuel is left in the system can aid in planning for subsequent flights, as well as in emergency procedures should they become necessary.

The other options relate to different aspects of fuel management but do not accurately define residual fuel. Fuel consumed during engine operation denotes the fuel actively used for propulsion and thus would not remain in the tanks. Vaporized fuel refers to the state of the fuel but does not signify what remains in the tank after shutdown. Fuel mixed with oil is a completely different process often seen in two-stroke engines and is not applicable to aviation fuel systems.