The term LASER is the abbreviation for the English term Light Amplification by Stimulated Emission of Radiation.  
Here, the light is concentrated in a specific wavelength, dependent on the medium through which the laser is passing.

The medium may be a gas, crystal or an electrical medium such as diodes.

In human medicine, depending on the medical treatment objective, the physical properties of the laser are selected to achieve the desired biological results through interaction with the tissue.
For this purpose, the laser beam, with the help of different laser probes, is applied in a minimally invasive way to the diseased tissue.