coherence, constant phase difference in two or more Waves over time. Two waves are said to be in phase if their crests and troughs meet at the same place at the same time, and the waves are out of phase if the crests of one meet the troughs of another. The waves are incoherent if the crests and troughs meet randomly. Coherence underlies a variety of physical phenomena, such as interference and diffraction. Coherence is also responsible for many of the remarkable properties of laser radiation; laser light is coherent, which is to say that the light waves from a laser are all in phase.