The rate of change of the total phase shift with respect to angular frequency, d/d, through a device or transmission medium, where is the total phase shift, and is the angular frequency equal to 2f, where f is the frequency. 2. In an optical fiber, the transit time required for optical power, traveling at a given mode's group velocity, to travel a given distance. Note: For optical fiber dispersion measurement purposes, the quantity of interest is group delay per unit length, which is the reciprocal of the group velocity of a particular mode. The measured group delay of a signal through an optical fiber exhibits a wavelength dependence due to the various dispersion mechanisms present in the fiber.