The word you're looking for is propellant. In chemical rockets, the fuel and propellant are the same thing (to be precise, the combustion byproducts of the for

The word you're looking for is propellant. In chemical rockets, the fuel and propellant are the same thing (to be precise, the combustion byproducts of the former comprise the latter). In nuclear fusion and ion thrusters, they're different. In all cases, Newton's second law means that you are using up propellant to generate an equal and opposite reaction in the direction you want to go, and thus the amount of "going" (delta-v) you can do is limited.