A propellant was a substance used to create thrust or another motion-producing force on an object, such as in the propulsion of a vehicle or projectile. Types of propellant include hypergolic propellant and monopropellant. Fuel was sometimes used as a propellant or used to produce a propellant.
Synthetic moyerite was a semi-solid propellant. (TNG: "Night Terrors", okudagram)
Crystalline emiristol was a solid propellant. The Kataan people used this propellant as fuel in the Kataan probe. (TNG: "The Inner Light")