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.
Propellants vary in the energy they can provide; the highest energy fuels are in space launch vehicles or weight-limited military unguided missiles and rocket engines. The term propellant is also used for gunpowder in firearms; modern smokeless powders are variously called single-base, double-base or triple-base propellant.
They have different storage lives, from hours to supercooled liquid hydrogen, to years for some stored chemicals. Experienced diesel fuel engineers do not assume a tank will last indefinitely, if only from water condensation; for both fuel turnover and engine lubrication, it is wise to run a backup diesel generator weekly.
Liquid propellants may be pressurized in simple engines, but usually need a fuel pump.