Refers to the path taken by an elastic impulse in a material (transverse, torsional or longitudinal displacement) before reflection. Any discontinuity such a crack or inclusion alters the impedance of the material and hence creates a reflected wave which can be seen on an instrument such as an oscilloscope. Reflection occurs naturally at the material/air boundary, which is why a rod or pipe generates a characteristic tone when you hit it. The speed of the impulse in a pure sample is known, so given the frequency of the timebase and the position of the reflected "blip", you know where the defect is.