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 such as ISO Guide 17025.
The most basic requirement of a calibration is proof of traceability. Traceability is defined as an unbroken chain of comparisons, all having stated uncertainties, between your measurement and some national or internationally accepted standard.