Propellant

In the broadest sense, a propellant is a source of gas, or sometimes liquid, for moving a powered object. Monopropellants need no other chemicals; they can be as simple as the air released from an inflated party balloon, or as complex as the precise flow pumps of an engine conveying monopropellant to a catalyst chamber. In another usage, a propellant is synonymous with fuel, in that it is combined with external air or an internal oxidizer to burn and produce gas. Solid rocket propellants may refer to the fuel component alone, or the entire mixture of fuel, oxidizer and binder.

Few propellants are better known than gasoline or diesel fuel in an automotive engine, oxidized by atmospheric oxygen.