The velocity of propagation of an envelope produced when an Electromagnetic Wave is modulated by, or mixed with, other waves of different frequencies.

Note: The group velocity is the velocity of information propagation and, loosely, of Energy propagation.

2. In optical Fiber transmission, for a particular mode, the reciprocal of the rate of change of the Phase Constant with respect to angular frequency.

Note: The group velocity equals the Phase Velocity IF the phase constant is a linear function of the angular frequency, = 2f , where F is the frequency.

3. In optical-fiber transmission, the velocity of the modulated optical power.