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propellant
This term carries a strong technical and industrial connotation, evoking images of high-pressure combustion and rapid acceleration. It is almost exclusively used in aerospace, ballistics, or chemical engineering contexts, rather than in casual conversation about movement. While a fuel provides energy, a propellant specifically emphasizes the act of expulsion to create thrust. It suggests a violent or controlled release of energy that forces an object in the opposite direction of the exhaust.
Countable when referring to a specific type of chemical agent (a new propellant). Uncountable when referring to the substance as a mass of material (the tank is full of propellant).