Sensor is defined as an element which produces signal relating to the quantity being measured.
According to the Instrument Society of America, sensor can be defined as “A device which provides a usable output in response to a specified measurand.” Here, the output is usually an ‘electrical quantity’ and measurand is a ‘physical quantity, property or condition which is to be measured’.
Thus in the case of, say, a variable inductance displacement element, the quantity being measured is displacement and the sensor transforms an input of displacement into a change in inductance.