a device containing a substance the majority of whose atoms or molecules can be put into an excited energy state, allowing the substance to emit coherent light of a precise wavelength in an intense, narrow beam
See laser in American Heritage Dictionary 4
(lāˈzər)
noun
Any of several devices that emit highly amplified and coherent radiation of one or more discrete frequencies. One of the most common lasers makes use of atoms in a metastable energy state that, as they decay to a lower energy level, stimulate others to decay, resulting in a cascade of emitted radiation.