Useful load

Fuel, any other fluids that are not part of empty weight, passengers, baggage, pilot, copilot, and crewmembers. It is determined by subtracting the empty weight from the maximum allowable gross weight.

Aviation Maintenance Technician Handbook — General, p. G-11

Also defined in 3 more sources

(GAMA) The difference between takeoff weight, or ramp weight if applicable, and basic empty weight.

Aircraft Weight and Balance Handbook, p. G-5

The difference between the gross weight and the basic empty weight. It includes the flight crew, usable fuel, drainable oil, if applicable, and payload.

Helicopter Flying Handbook, p. G-6

The weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil. It is the basic empty weight subtracted from the maximum allowable gross weight. This term applies to general aviation aircraft only.

Pilot's Handbook of Aeronautical Knowledge, p. G-32