Basically, a deactivated missile. Typically, an inert missile will either be a purpose built dummy, or a live missile, with its propellant and warhead removed.
missile
Inert means they don't react. Group 18, the noble gases, are inert.
An air-to-surface missile is a missile which is fired from an aircraft to the ground.
inert as in the inert gases or "noble" gases
Missile fuel
The term is "inert." Inert substances do not react chemically with other substances.
The inert gas had no effect on the experiment. The bomb was inert and failed to explode.
Carbon monoxide is not an inert gas.
Helium is an inert gas, and seldom reacts with other elements. After the explosion, they discovered the inert bodies of the two guards. The inert Lunar Rover is still parked at the landing site of Apollo 17, awaiting a new battery and a new driver.
The standard term for a missile chamber (be it a ballistic missile, vertical launch cruise missile, torpedo, etc.) is simply a tube - straight and to the point.
Inter continental bellestic missile
antitank missile