Final answer:
The wave traveling between the probes at the test surface is known as the lateral wave, which is different from other types of waves such as the backwall signal, tip diffraction signal, and discontinuity signal.
Step-by-step explanation:
The wave traveling between the probes at the test surface is called the lateral wave. When a wave propagates on a test surface, different types of waves can be generated depending on the material and the angle of incidence. The lateral wave travels along the surface of the material between the probe and the edge or between two probes. It is distinct from the backwall signal, which reflects off the far side of the material being tested, the tip diffraction signal, which arises from the tip of a crack or flaw diffraction, and the discontinuity signal, which indicates the presence of an anomaly or discontinuity within the material.