Source : Webster's Revised Unabridged Dictionary (1913)

Calibration \Cal`ibra"*tion\, n.
   The process of estimating the caliber a tube, as of a
   thermometer tube, in order to graduate it to a scale of
   degrees; also, more generally, the determination of the true
   value of the spaces in any graduated instrument.

Source : WordNet®

calibration
     n : the act of checking or adjusting (by comparison with a
         standard) the accuracy of a measuring instrument; "the
         thermometer needed calibration" [syn: {standardization},
         {standardisation}]