A propellant is a material that produces pressurized gas that:

• can be directed through a nozzle, thereby producing thrust (rocket propellant used in a rocket motor);
• fills the interior of an ammunition cartridge or the chamber of a gun or cannon, leading to the expulsion of a bullet or shell (gunpowder, smokeless [powder, and large gun propellants);
• can fill an expansible bag or membrane, such as an automotive airbag (gas generator propellants).

Common chemical propellants are energetic materials and consist of a fuel; like gasoline, jet fuel, rocket fuel and an oxidizer. Chemical propellants are burned or otherwise decomposed to produce the propellant gas. Other propellants are simply liquids that can readily be vaporized into the desired pressurized gas. Such propellants include the nitrouls oxide that is dissolved in the butterfat canned whhipped cream and the dimethyl ether or low-boiling alkane used in hairspray.

Propellants are used in pressurised dispensing systems, such as aerosol sprays, to force a material through a nozzle.

In rockets, the propellant produces an exhaust and the exhausted material is usually expelled under pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid or gelled.

In aircraft, the propellant is usually a fuel and is combusted to generate energy to power the aircraft's flight.

(Wikipedia, Propellants, 10/26/2011)

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