A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word laser is an acronym that stands for "light amplification by stimulated emission of radiation". Lasers work by exciting electrons in atoms or molecules in optical materials like glass, crystal, or gas, causing them to move from a lower-energy orbit to a higher-energy orbit around the atom’s nucleus. When these electrons return to their normal orbit, they emit photons of light. The photons in a laser beam are coherent, meaning they are moving in the same direction at the same wavelength.

Laser science or laser physics is a branch of optics that describes the theory and practice of lasers. It is principally concerned with quantum electronics, laser construction, optical gain, and nonlinear optics. Laser science predates the invention of the laser itself, with Albert Einstein creating the foundations for the laser and maser in 1917. The first working laser was demonstrated on May 16, 1960, by Theodore Maiman at the Hughes Research Laboratories.

Lasers have many different types, with highly varied characteristics. Some lasers, such as ruby lasers, emit short pulses of light, while others, like helium–neon gas lasers or liquid dye lasers, emit light that is continuous. Lasers can be used in a wide range of applications, including in medicine, telecommunications, manufacturing, and entertainment.