Calibration is defined as a set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by a measuring instrument or measuring system … and the corresponding values realised by standards.

Sometimes, however, the word calibration is misused to describe the process of altering the performance of an instrument to ensure that the values it indicates are correct within specified limits (eg adjusting a instrument until its reading agrees with that of another instrument). Strictly this is adjustment - defined as the ... operation of bringing a measuring instrument into a state of performance suitable for its use - and not calibration, although the nature and magnitude of the adjustment is often determined by a pre-adjustment calibration, sometimes known as an as found calibration; such a procedure thus runs:

• first calibration (to determine the approximate magnitude of the adjustment needed)
• adjustment
• second calibration.

Results from the first calibration will also be needed if the device's reproducibility is to be estimated (essentially its change in characteristics over a long time interval).