A calibration is a comparison between a known reference value, called a standard, and an unknown value produced from the examined device. In most cases the known standard value is used to stimulate a response from the equipment being calibrated, and that response from the equipment under test is then compared to the stimulus value. The observed difference between the standard and the device under test is the result of the calibration.
NOTE: It is important not to confuse CALIBRATION with the term "calibrate" that is sometimes used on instruments to refer to the actions of setting up a transducer such as a loadcell or extensometer to work with the system.