In physics, a laser is a device that emits light through a specific mechanism for which the term laser is an acronym: light amplification by stimulated emission of radiation. This is a combined quantum-mechanical and thermodynamical process discussed in more detail below. As a light source, a laser can have various properties, depending on the purpose for which it is designed. A typical laser emits light in a narrow, low-divergence beam and with a well-defined wavelength (or color, when the laser is operating in the visible spectrum). This is in contrast to a light source such as the incandescent light bulb, which emits into a large solid angle and over a wide spectrum of wavelength. These properties can be summarized in the term coherence.

A laser consists of a gain medium inside an optical cavity, with a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium. Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The process of supplying the energy required for the amplification is called pumping and the energy is typically supplied as an electrical current or as light at a different wavelength. In the latter case, the light source can be a flash lamp or another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories. Nowadays, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in optical storage devices such as compact disc and dvd players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers and laser pointers. In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are also used for military and medical applications.