A cold gas thruster is a rocket engine/thruster that uses a (typically inert) gas as the reaction mass. A cold gas thruster is the simplest case of a thermal rocket, in which the thermal source is the thermal energy in the form of the heat capacity of the gas.
A cold gas thruster usually consists of simply a pressurized tank containing gas, a valve to control its release and a nozzle, and plumbing connecting them. A very simple example would be the use of a handheld CO2 or nitrogen gas fire extinguisher while sitting down in a rolling office chair; motion is achieved by pointing the nozzle in the direction opposite of the desired movement and activating the extinguisher.
Because the gas is usually unheated, speed at the throat is low and very low performance is achieved; in a vacuum with nitrogen gas a specific impulse of 73 seconds can be achieved. The maximum theoretical specific impulse for nitrogen gas is 76 seconds. In the simplest approximation, the specific impulse is modelled as proportional to the square root of the (absolute) gas temperature, so performance rises as the gas temperature is increased. A thruster in which the performance is increased by heating the gas by an electrical resistance is known as a resistojet.
Cold gas thrusters are mostly useful for vernier engines, and are employed chiefly for simplicity and reliability.