Calibration or re-certification is a measurement and/or adjustment (if required) of various device parameters to
ensure its accuracy within prescribed standards or device specifications. These specifications are usually those prescribed by the OEM of the product or device being re-certified for calibration. 
The calibration is a comparison (quantitative numbers) between measurements - one of known setting or set with one device output and another measurement made in as similar a way as possible with a second device under test (e.g. an automated or manual test instrument).
To improve the quality of the calibration and have the results accepted by outside organizations it is desirable for the calibration and subsequent measurements to be "traceable" to the internationally defined measurement units. Establishing traceability is accomplished by a formal comparison to a standard which is directly or indirectly related to national standards ((e.g. National Institute of Standards and Technology (NIST) in the USA)), international standards, or certified reference materials.