The term "laser" is an acronym for "optical amplification by stimulated emission of radiation". It is a mechanism that emits electromagnetic radiation through a stimulated emission process. The laser was the first device capable of amplifying the light wave itself. The emitted laser light is a spatially coherent narrow and low divergence beam. When the waves (or photons) of the beam have the same frequency, phase, and direction, they are said to be coherent. Some lasers emit light with a broad spectrum, or at the same time emit light of different wavelengths. According to the Encyclopedia of Laser Physics and Technology, the beam divergence of a laser beam is a measure of the speed at which the beam expands from the waist of the beam. Laser beams with narrow beam divergence are widely used in manufacturing laser pointer devices. Generally, a beam profiler is used to measure the beam divergence of a laser beam.
In electromagnetics, especially in optics, beam divergence is a measure of the angle in which the beam diameter or radius increases with the distance from the optical aperture or antenna aperture of the emitted beam. The term is only useful in the "far-field," away from any focus of the beam.
The Gaussian beam is an electromagnetic radiation beam whose lateral beam and intensity (diffracted illuminance) distribution is described by a Gaussian function.
The beam parameters control the behavior and geometry of the Gaussian beam. The important beam parameters are described in the beam divergence angle.
However, when the beam spreads for extended periods of time, it will slowly diverge or spread out. For electromagnetic beams, beam divergence is the angular angle at which the transmitted beam appears, and the size or diameter increases with the distance from the optical aperture.