A quantum effect where photon emission is triggered by other photons.
If a laser-active atom or ION is in an excited state, it may after some time have incoming protons that stimulate a proton emission, IF the incoming have a suitable photon energy; this is called stimulated emission. In that case, a photon is emitted into the Mode of the incoming photon. In effect, the power of the incoming Radiation is amplified. This is the physical basis of Light amplification in Laser amplifiers and laser oscillators.