Determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors

The set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument or system and the corresponding known values of the measurement.

What is the purpose of calibration? 

The purpose of calibration is to ensure that a measuring device is accurate, reliable and consistent. Any kind of measurement is used in a decision. making process, so it is essential that the data collected is accurate.

Otherwise,conclusions are drawn based on errors, and, in those cases, decisions are sometimes wrong and have negative consequences. With a well calibrated device, the margin of error is known and taken in to consideration.