any gas, liquid, or solid the expansion of which can be used to impart motion to another substance or object. In aerosol (aerosol container) dispensers, compressed gases such as nitrous oxide, carbon dioxide, and many halogenated hydrocarbons (halocarbon) are used as propellants. The propellant may remain in gaseous form (nitrous oxide or carbon dioxide), or it may liquefy under pressure. Food (food processing) products, such as artificial whipped cream, are propelled by nitrous oxide or carbon dioxide; nonfood products, such as cosmetics, insecticides, paints, and pharmaceuticals, formerly were dispensed with the aid of fluorinated hydrocarbons. Because of the threat believed to be posed to the Earth's ozone layer by halogenated propellants, they have been banned in many countries except for essential uses such as some drugs, pesticides, lubricants, and cleaners for electrical or electronic equipment. (See also aerosol container.)

Solid and liquid propellants are substances that undergo rapid combustion, producing gaseous products. black powder was used as a propellant in guns and rockets (rocket) until the 20th century, when double-base gunpowder (40 percent nitroglycerin, 60 percent nitrocellulose) came into use. Other modern solid propellants are cast perchlorate (using perchlorate as oxidizer and various oils or rubbers as fuel) and composite propellants (using a plastic binder with ammonium picrate, potassium nitrate, or sodium nitrate). There are various liquid rocket propellants: monopropellants, such as nitromethane, which contain both oxidizer and fuel and are ignited by some external means; bipropellants, consisting of an oxidizer such as liquid oxygen and a fuel such as liquid hydrogen, which are injected into a combustion chamber from separate containers; and multipropellants, consisting of several oxidizers and fuels.