An acronym for “light amplification by stimulated emission of radiation.” Rather than producing light of varying wavelengths that scatters in every direction, as a light bulb does, the laser apparatus creates light that is coherent – meaning that only one wavelength is created and every photon (“packet”) of light travels in precisely the same direction. Somewhat confusingly, “laser” may be used to designate both the machine that produces the laser beam, and the beam itself.

See also: laser surgery.