A laser (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam. The back-formed verb lase means "to produce laser light" or "to apply laser light to".

In analogy with optical lasers, a device which produces any particles or electromagnetic radiation in a coherent state is also called a "laser", usually with indication of type of particle as prefix (for example, atom laser.) In most cases, "laser" refers to a source of coherent photons, i.e. light or other electromagnetic radiation.

Laser light is typically near-monochromatic, i.e., consisting of a very small band of wavelengths or a very specific color, and emitted in a narrow beam. This contrasts with common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths.

Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.

The laser was proposed as a variation of the maser principle in the late 1950s, and the first laser was demonstrated in July 1960 by Theodore Maiman at Hughes Research Laboratories. Since that time, laser manufacture has become a multi-billion dollar industry, and the laser has found applications in fields including science, the defense industry, medicine, and consumer electronics.