A laser is a device which emits light which is amplified by stimulated emission. The word stands for light amplification by stimulated emission of radiation and derives from maser, a similar and earlier invention which emits microwaves. Lasers were originally called optical masers.
Lasers are available in a wide range of wavelengths, except for wavelengths which can not be effectively reflected with current technology (such as gamma rays). Stimulated emission is still possible at such wavelengths, but the intensity of the beam is lower because the effective feedback loop from the mirrors is not present.
Principle of operation
An energy source causes light to be emitted within a substance. The energy source places outermost electrons in the substance in a high energy state. That light is bounced back and forth between mirrors, causing it to pass through the lasing substance again and again. Whenever a photon strikes one of the atoms, the outer electron drops to a lower energy state and emits two photons (equivalent to the energy of the photon which stimulated it plus the energy released by the state change).
In this manner, a very large number of photons are emitted at wavelengths corresponding to the energy difference between the ground state of the outermost electrons and the higher energy state. Some of these photons continue to stimulate the lasing process, while others escape through a half-mirror in an intense beam of light.
The intensity of the beam is in part due to the fact that all of the photons at the same wavelength are approximately in phase. (A laser may be monochromatic but it can also emit light at more than one wavelength, depending on the lasing substance.)