Calibration is the comparison of an instrument's actual measurement performance to a standard of known accuracy. Calibration performed on an instrument can document the deviation of a measurement from the known standard, or it can include adjusting the instrument's measurement capability to improve measurement accuracy. International committees define standards for calibration.
The most basic requirement of a calibration is the proof of traceability. Traceability is defined as an unbroken chain of comparisons, all having stated uncertainties between your measurement and some nationally or internationally accepted standard