A general name for enhanced radiation. Examples occur in quantum mechanics, astrophysics, and relativity theory.
In quantum mechanics, an example is given by a set of N two-level atoms that has a laser beam of monochromatic radiation interacting with the atoms. If the wavelength of the light from the laser is much smaller than the distance between the atoms, then there are no collective effects and the intensity of the emitted light is proportional to N; however, if the distance between atoms is small compared with the wavelength, then superradiance (i.e. collective spontaneous emission) occurs and the intensity is proportional to N2.
Another example of superradiance occurs in general relativity theory in what can be regarded as a wave analogue of the Penrose process. This superradiance can occur when electromagnetic waves or gravitational waves are sent towards a rotating black hole. For certain frequencies it is possible that the part of the wave that is absorbed by the black hole carries negative energy, with the result that, contrary to the normal situation, the part of the wave that is not absorbed has greater energy and intensity than the original wave.