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Most so-called single wavelength lasers actually produce radiation in several modes having slightly different frequencies (wavelengths), often not in a single polarization. And although temporal coherence implies monochromaticity, there are even lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously. There are some lasers which are not single spatial mode and consequently their light beams diverge more than required by the diffraction limit. However all such devices are classified as lasers based on their method of producing that light: stimulated emission. Lasers are employed in applications where light of the required spatial or temporal coherence could not be produced using simpler technologies.

The word laser started as an acronym for light amplification by stimulated emission of radiation ; in modern usage light broadly denotes electromagnetic radiation of any frequency, not only visible light, hence infrared laser, ultraviolet laser, X-ray laser, and so on. Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave and radio frequencies are referred to as masers rather than microwave lasers or radio lasers . In the early technical literature, especially at Bell Telephone Laboratories, the laser was called an optical maser; this term is now obsolete.[4]

A laser which produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for light oscillation by stimulated emission of radiation, would have been more correct.[5] With the widespread use of the original acronym as a common noun, actual optical amplifiers have come to be referred to as laser amplifiers , notwithstanding the apparent redundancy in that designation.

The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flashp lamp or perhaps 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.