Laser

1. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.
2. Laser is a device that utilizes the natural oscillations of atoms or molecules between energy levels for generating a beam of coherent electromagnetic radiation usually in the ultraviolet, visible, or infrared regions of the spectrum.

Lasers can be made to run continuously with a single well-defined frequency and wavelength, with an accuracy better than one part in 1016. This enables laser light to provide a standard for time and length scales. It provides a source so spectrally pure that it can be tuned to very narrow resonances in atoms and molecules, coupling so strongly that the internal states of the atoms and molecules can be manipulated with great precision, and allowing their thermal motion to be slowed so much that their temperature can be reduced to within a small fraction of a degree of absolute zero. Alternatively, they can be made to give extremely short pulses (less than 10-15 seconds long) which can capture ultrafast processes such as chemical reactions; such laser pulses intrinsically produce a broad spectrum of wavelengths all at the same time. Lasers can be made with beams that hardly diverge, so that their reflection from the mirrors placed on the moon by Apollo 11 can be measured on earth, or they can be made to uniformly illuminate large regions for manufacturing applications. They can be powerful enough to induce nuclear fusion, precise enough to replace scalpels, or gentle enough to reduce heating in a high speed computer processor. The electric and magnetic waves they produce can be very simple, or can be structured into astonishingly complex patterns (both in space and in time) for applications such as optical communications, medical imaging, and controlling chemical reactions. Both the unique properties and the flexibility of the laser make it an important part of modern life.

Sources:

Англійський словник Коллінза, https://www.collinsdictionary.com/

MacMillan Dictionary https://www.macmillandictionary.com/dictionary/british

LaserFest https://laserfest.org/lasers/how/laser.cfm