Coherent Light is light with both
a constant phase difference and Frequency,
i.e., consisting of Coherent waves.
Lasers (the Visible Light analog
of Masers) produce such light.
One 1995 astronomical observation has been interpreted as a
Natural Laser, but Masers are more common.
Lasers are also used for Adaptive Optics, in detectors for
Gravitational Wave, and some high energy Electromagnetic Radiation, and in carrying out
particle physics experiments of Cosmological significance.