Calibration:

The set of operation which establish, under specified conditions, the relationship between values indicated by measuring instrument or measuring system, or values represented by a material measure, and the corresponding known value of a measurand.

Measurand:

A quantity subjected to measurement.

Error:

An error is something which pertains to a measurement and not to an instrument. An error is the difference between the value obtained on the basis of a set of measurements and the conventional true value of the quantity measure.

Uncertainty:

The uncertainty of a measurement is the range about the measured value within which the true value or the conventional true value of the measured quantity is likely to lie at the stated level of confidence.

Repeatability:

It is the ability of the measuring instrument to give identical indications or responses for repeated applications of the same value of the measured quantity, under stated conditions of use.

Accuracy of Instrument:

The accuracy of an instrument is its ability to give correct results. The accuracy and repeatability are two different properties of an instrument. Accuracy is a measure of an instrument’s ability to tell the truth, while repeatability is a measure of its ability to indicate the same value of the measured quantity. Consequently, good repeatability is not a guarantee of good accuracy. Although poor repeatability is sure sign of poor accuracy.

Flatness:

The maximum permissible distance separating two imaginary parallel planes with in which the surface under consideration can just be enclosed.