It is defined as: "Property of a measurement result relating the result to a stated metrological reference (free definition and not necessarily SI) through an unbroken chain of calibrations of a measuring system or comparisons, each contributing to the stated measurement uncertainty."^[1]

Traceability can be also defined as: "the ability to chronologically interrelate the uniquely identifiable entities in a way that is verifiable. Traceability is the ability to verify the history, location, or application of an item by means of documented recorded identification"^[2]

The term traceability is used to refer to an unbroken chain of comparisons relating an instrument's measurements to a known standard. Calibration to a traceable standard can be used to determine an instrument's bias, precision, and accuracy.

In many countries, national standards for weights and measures are maintained by a National Measurement Institute (NMI) which provides the highest level of standards for the calibration / traceability infrastructure in that country. Examples of government agencies are the National Physical Laboratory, UK (NPL) the National Institute of Standards and Technology (NIST) in the USA, Physikalisch-Technische Bundesanstalt (PTB) in Germany. As defined by NIST, "Traceability requires the establishment of an unbroken chain of comparisons to stated references each with a stated uncertainty."^[3]

References

[1] ISO guide 99 Vocabulary for International Metrology

[2] "Glossary," ASME Boiler and Pressure Vessel Code, Section III, Article NCA-9000

[3] National Institute of Standards and Technology (NIST) - www.nist.gov/index.html