From AMS Glossary
(Acronym for light amplification by stimulated emission of radiation.) A device that produces a narrow beam of electromagnetic energy by recirculating an internal beam many times through an amplifying medium, each time adding a small amount of energy to the recirculating beam in a phase-coherent manner.
Typically the output beam results when a small amount of recirculating energy is allowed to leak out from the internal "cavity." These devices produce energy at light frequencies (which are higher than radio frequencies) in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum, and hence they often use optical technologies.