What is "thermal rocket"

n. a rocket that is powered by a thermal source, such as a nuclear-thermal rocket or a solar-thermal rocket or a rocket powered by an energy beam such as a laser or microwave beam

A thermal rocket is a rocket engine that uses a propellant that is externally heated before being passed through a nozzle, as opposed to undergoing a chemical reaction as in a chemical rocket.

Thermal rockets can theoretically give high performance, depending on the fuel used and design specifications, and a great deal of research has gone into a variety of types. However, aside from the simple cold gas thruster and steam rocket, none has yet been put to practical use.