Fuel/ˈfjuːəl/ Definition 1 of 3Pro Tip 1/3Types of Fuel Fuels vary from liquids to gases and solids. Know the right type for the context. Wood is a solid fuel used in campfires. Pro Tip 2/3Fuel Usage Talk about fuel in context of machines, heaters, engines, not living things. The truck's engine requires diesel fuel. Pro Tip 3/3Measurement Units Use liters or gallons to measure liquid fuel; context depends on the country. I need ten gallons of fuel for my boat.