1. The surface over which the phase of a traveling wave disturbance is the same. The wavefront moves perpendicular to itself as the disturbance travels in an isotropic medium. A locus of equal traveltime. 2. The leading edge of a waveform. A wavefront is a phase velocity surface, the locus of phase propagation. S-wave group velocity may have cusps when S-anisotropy exceeds 10%.