A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation. The emitted laser light is notable for its high degree of spatial and temporal coherence.
Spatial coherence is typically expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can be launched into beams of very low divergence in order to concentrate their power at a large distance.
Temporal (or longitudinal) coherence implies a polarized wave at a
single frequency whose phase is correlated over a relatively large
distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length.
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.
