TIA's Glossary of Telecommunication Terms
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coherence time
For an electromagnetic wave, the time over which a propagating wave may be considered coherent. (188) Note 1: In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction. Note 2: In optical communications, coherence time, 
, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by = 
2/(c
) where is the central wavelength of the source, is the spectral width of the source, and c is the velocity of light in vacuum. Note 3: "Coherence time " is usually applied to the optical regime.