Laser: a device that produces a beam of high-energy light

Laser stands for "light amplification by stimulated emission of radiation." Inside a laser is a material called a lasing medium. Passing an electric current or light into the medium gives energy to, or excites, its atoms. The excited atoms suddenly give up their extra energy and emit light. One atom emits a light ray, which strikes another atom and causes it to emit another ray, and so on in a rapid cascade of emissions. The rays are all in phase. This means that the waves of energy in the rays are exactly in step, making the light very concentrated. This kind of light is coherent radiation. Mirrors reflect the rays, so that the cascade builds up. The light leaves through one of the mirrors, which is partly transparent. Lasers can also emit invisible infrared rays. A maser is like a laser, but emits microwaves. (Dictionary of Science, by Neil Ardley)