(This could be a device to convert the temperature of the surrounding air or water into the visually readable height of a column of mercury in a small tube, for example; but the conversion could also be made to an electronic encoding of the parameter, for reading by a computer system.)
It is the relation of the output to the input over a range expressed as a percentage of the full-scale measurements.
In the context of a digital-to-analog converter (DAC) or an analog-to-digital converter (ADC), independent linearity is fitted to minimize the deviation with respect to the ideal behaviour with no constraints.
For rotary position sensors, as a shaft (or in the case of magnetic sensors, a magnet) is turned over a defined mechanical range in a direction causing an increasing response, an output voltage changes from a minimum to maximum value.
It is measured in a practical sense as deviation of output voltage as a percentage of input voltage with the maximum value as the range is traversed, usually being referred to in a device's specifications.