Introduction
The laser is a device for producing electromagnetic waves of the same nature as light waves, radio waves and X-rays but which have characteristics not present in any other natural or man-made radiation available at present. 
The characteristics of laser light are essentially coherence which is the coincidence of phases both in time, in which case we speak of monofrequency or monochromaticity, and in space for which it is possible to obtain extremely concentrated light beams even for paths kilometres long or to focalize the beams or make them diverge in an extremely controlled manner.
The difference between sunlight or normal incandescent light used for domestic illumination and laser light is that the sun or the lamp emit radiation or quanta in a random fashion, like people leaving a stadium or cinema, while the laser emits extremely orderly quanta of energy, like a platoon of soldiers or robots, all exactly alike, that march perfectly aligned in rows and columns. 
With a rhetorical figure of synecdoche, the word laser has come to identify the apparatus in which the physical laser process occurs in a controlled form. 

Historical notes
The first applications in the field of medicine were in ophthalmic pathology, while in 1962 Goldman applied the laser in field of dermatology. G.L. Jako used a CO₂ laser with a microscope for surgical applications on the vocal chords. The first Authors to report results of surgical applications of a CO₂ laser in gynaecology are I. Goldman, I. Kaplan, R. Ger.
The assumptions for the discovery of the laser were proposed by the development of quantum physics. The bases of this theory date back to the early 1900s, from Plancks's studies of radiation of the black body and developed through the atom model with discrete energy levels attributed to Bohr, Einstein's theory (1917) of stimulated emission of photons and to Einstein and Bose's data on photons. 

In 1950 Lamb and Rutherford performed the first important experimental verification of a mechanism essential for laser function: the population inversion in the energy levels in an atomic or molecular system. Block theoretically predicted such a mechanism during his work in 1945. In 1955 Lamb received the Nobel Prize in Physics for his important work in this field. In 1964 the Nobel Prize for Physics was again awarded to researchers who contributed to the birth of the laser: an American, Towens, and two Russians, Basov and Prochorov, who since 1951 had been involved in research to create quantum amplifiers by means of systems placed in a condition of energy state population inversion.

Important research on this topic was also performed by the American, Weber during the same period. The first working apparatus was constructed by the Towens group at Columbia University in 1954. For the first time they were able to obtain radio waves at a microwave frequency without the use of the usual electronic tube. The waves, little more than a centimetre in length, were generated in a small metal box that contained only ammonia gas. The "Maser", antecedent of the laser, was born. "Maser" is the abbreviation of "Microwave Amplification by Stimulated Emission of Radiation". Following this, (1958) the principle was extended by Towens and Schawlow to the field of optical frequencies, and the letter "M" came to indicate "molecular", while "amplification" at optical frequencies became known as "laser" from the acronym of "Light Amplification by Stimulated Emission of Radiation". 

In mid-1960, T.H. Maiman, of Hughes Aircraft Co. USA, constructed the first optical Maser, or ruby laser, the first solid state laser. At the end of 1960, A. Javan and collaborators constructed the first gas laser using a mixture of helium and neon (He-Ne). In 1965 Patel created the first molecular gas (CO₂) laser.