Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy. Most designs are rocket engines where the energy is provided by the beam, and is used to superheat propellant that then provides propulsion, although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust.
The beam would typically either be a beam of microwaves or a laser. Lasers are subdivided into either pulsed or continuous beamed.
The rule of thumb that is usually quoted is that it takes a megawatt of power beamed to a vehicle per kg of payload while it is being accelerated to permit it to reach low earth orbit.
Other than launching to orbit, applications for moving around the world quickly have also been proposed.
Read more about Beam-powered Propulsion: Background, Thermal Propulsion, Electric Propulsion, Direct Impulse, Non-spacecraft Applications