In eddy current testing, the term "lift off" refers to:

In eddy current testing, "lift off" specifically refers to the separation between the test coil and the surface being examined. This separation can significantly affect the eddy current response measured by the system.

When a test coil is positioned at a certain distance from the test surface, the amplitude and phase of the eddy currents generated in the conductive material change due to the distance. As the lift off increases, the interaction between the coil and the test material diminishes, leading to a reduction in the induced eddy currents. Consequently, this can impact the readings and overall sensitivity of the inspection results. Understanding and accurately measuring lift off is critical for interpreting test data effectively, as variations can complicate the identification of defects or characteristics in the material being tested.

The other concepts mentioned in the other choices relate to different aspects of eddy current testing but do not define "lift off." Therefore, recognizing lift off as the distance between the coil and surface is essential for proper application and analysis in eddy current testing.