LASER: An acronym of Light Amplification by Stimulated Emission of Radiation. A  laser is a cavity that has mirrors at the ends and is filled with lasable material such
as crystal, glass, liquid, gas, or dye. These materials must have atoms, ions, or molecules capable of being excited to a metastable state by light, electric discharge,
or other stimulus. The transition from this metastable state back to the normal ground state is accompanied by the emission of photons which form a coherent beam.