Calibration is the comparison of a measurement device (an unknown) against an equal or better standard. A standard in a measurement is considered the reference; it is the one in the comparison taken to be the more correct of the two. One calibrates to find out how far the unknown is from the standard.
A "typical" commercial calibration references a manufacturer's calibration procedure and is performed with a reference standard at least four times more accurate than the instrument under test.
Calibration is an insurance policy.
Some people consider calibration a necessary annoyance to keep the auditor off their back. In fact, out of tolerance (OOT) instruments may give false information leading to unreliable products, customer dissatisfaction and increased warranty costs. In addition, OOT conditions may cause good products to fail tests, which ultimately results in unnecessary rework costs and production delays.