A propellant is a substance or substances used to furnish the exhaust material which, when allowed to escape from a rocket, provides thrust. A propellant consists of a fuel and an oxidizer. A fuel is a substance which burns when combined with oxygen producing gas for propulsion. An oxidizer is an agent that releases oxygen for combination with a fuel.
Most rocket engines use chemical propellants, which can be classified as liquid propellants, solid propellants, gelled propellants, hybrid propellants, or gaseous propellants depending on their physical state. Liquid propellants can be further subdivided into bipropellants and monopropellants. In an ion propulsion system the propellant particles are first ionized and then accelerated to yield a high-speed exhaust. The gauge for rating the efficiency of rocket propellants is specific impulse.