${\displaystyle \int \frac{1}{v_{\text{g}}}}\mathrm{d}s$

where v_g is the algebraic value of the projection of the group velocity vector onto the tangent to the ray path, and ds is the scalar line element

Note 1 to entry: In a homogeneous medium, or on a uniform transmission line, the group delay is equal to the derivative with respect to the angular frequency of the difference, at the same time, of the phases, at the two points, of the signal used to define the group velocity.

Note 2 to entry: In practice, if the medium is not too absorbent, nor too anisotropic, nor too dispersive, the group delay is equal to the propagation duration of the signal used to define the group velocity.