Is it ensure, insure, or assure?
1955, acronym from "microwave amplification (by) stimulated emission (of) radiation."
Short for microwave amplification by stimulated emission of radiation. A device that generates coherent microwaves using the same principles as a laser. Masers are used in a variety of applications, including in atomic clocks. Natural masers are found in outer space when water or other substances are excited by radiation from a star or by the energy of a collision.