What term defines the difference between the actual instrument output and the expected output as defined by a straight line calibration curve?

The term that defines the difference between the actual instrument output and the expected output as defined by a straight line calibration curve is nonlinearity. Nonlinearity occurs when the response of the instrument does not follow a linear correlation with the input or measurement being evaluated. In a perfectly linear system, the output would directly correlate with the input; however, if there are deviations or discrepancies from this expected outcome, it signifies nonlinearity.

Understanding nonlinearity is crucial because it can significantly affect the accuracy of measurements and interpretations in various testing scenarios. It indicates that the calibration curve is not perfectly straight and that different ranges of input may yield outputs that do not correspond to a simple linear relation, necessitating corrections or adjustments in the calibration process.

In contrast, other terms such as phase shift, lift-off, and skin effect represent different phenomena in measurement and testing scenarios, making them unsuitable for defining the specific calibration deviation described in the question.