propellant

A propellant is a substance, or a mix of substances, used to create thrust or pressure that pushes something forward. People often think first of rockets and spacecraft, and that is one of the main uses. In a rocket, the propellant is the material that is burned or expelled to produce the force that lifts the vehicle and moves it through the air or space.

In rocketry, propellants may be solid, liquid, or gas-based. Solid rocket fuel, for example, burns and releases hot gases. Liquid systems use stored materials, often a fuel and an oxidizer, that react and are forced through a nozzle. In each case, the fast-moving exhaust creates the push that sends the rocket in the opposite direction.

But the word is not limited to space travel. In firearms, a chemical charge such as gunpowder acts as the propellant by producing expanding gas that drives the bullet forward. In spray cans, a compressed gas serves as the propellant, forcing the paint, deodorant, or other contents out of the container. The same basic idea appears anywhere a stored substance provides the push.

There is also a figurative use, though it is less common. Someone might call anger, hope, or ambition the propellant behind an action or a movement. In that sense, the word means the thing that gives momentum and sends events forward.

So whether it is launching a rocket, firing a round, or pushing liquid out of a spray can, a propellant is the substance that provides the driving push. In figurative writing, it can also mean the motive force behind action.