A comparison between a device under test and an established standard, such as Coordinated Universal Time UTC (NIST). When the calibration is finished, it should be possible to state the estimated time offset and/or frequency offset of the device under test with respect to the standard, as well as the measurement uncertainty. Calibrations can be absolute or relative. Absolute calibrations are not biased by the calibration reference and would, therefore, be more reproducible. However, absolute calibrations can be more complex to determine. The bias in relative calibrations would be consistent if all the devices in the system are calibrated against the same calibration reference. Calibrations may also be performed relative to other devices without reference to an absolute standard. Relative calibrations are generally simpler to perform than absolute calibrations.
Sources:
NIST IR 8401 from NIST T&F Glossary - adapted