Maser

A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, the word "maser" is derived from the original upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation". The lower-case usage arose from technological development having rendered the original definition imprecise, because contemporary masers emit electromagnetic waves not just at microwave frequencies, but rather across a broader band of the electromagnetic spectrum. Hence, the physicist Charles H. Townes suggested the usage using "molecular" to replace "microwave" for contemporary linguistic accuracy.