मेसर एक ऐसा यन्त्र है जो उद्दीप्त उत्सर्जन द्वारा प्रवर्धन के माध्यम से सम्बंद्ध् विद्युतचुम्बकीय तरंगे उत्पन्न करता है। ऐतिहासिक रूप से यह परिवर्णी (संक्षिप्त) नाम मेसर (MESAR) से व्युत्पन्न होता है जिसका अर्थ विकिरण के उद्दीप्त उत्सर्जन द्वारा सूक्ष्मतरंग प्रवर्धन है।

A maser is a device that produces coherent electromagnetic waves (microwaves), through amplification by stimulated emission. The term is an acronym for microwave amplification by stimulated emission of radiation. First suggested by Joseph Weber, the first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953. Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical work leading to the maser. Masers are used as the timekeeping device in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes and deep-space spacecraft communication ground stations.