Definition:  the velocity vector, at a point in a propagation medium, of a signal which may be ideally represented by two superimposed sinusoidal waves of equal amplitude and slightly different frequencies approaching a common frequency limit
NOTE – In the most general case, the magnitude of the group velocity vector equals the derivative of the frequency with respect to the reciprocal of the wavelength, and the components of the group velocity along the coordinate axes equal the partial derivatives of the angular frequency with respect to the components of the real part $\overrightarrow{{K}^{\prime}}$ of the wave vector $\overrightarrow{\underset{\_}{K}}$.
