The unit definition file of GNU Units lists a large number of sources:
Most units data was drawn from
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI). Barry N. Taylor. 1995
- CRC Handbook of Chemistry and Physics 70th edition
- Oxford English Dictionary
- Websters New Universal Unabridged Dictionary
- Units of Measure by Stephen Dresner
- A Dictionary of English Weights and Measures by Ronald Zupko
- British Weights and Measures by Ronald Zupko
- Realm of Measure by Isaac Asimov
- United States standards of weights and measures, their creation and creators by Arthur H. Frazier.
- French weights and measures before the Revolution: a dictionary of provincial and local units by Ronald Zupko
- Weights and Measures: their ancient origins and their development in Great Britain up to AD 1855 by FG Skinner
- The World of Measurements by H. Arthur Klein
- For Good Measure by William Johnstone
- NTC's Encyclopedia of International Weights and Measures by William Johnstone
- Sizes by John Lord
- Sizesaurus by Stephen Strauss
- CODATA Recommended Values of Physical Constants available at http://physics.nist.gov/cuu/Constants/index.html
- How Many? A Dictionary of Units of Measurement. Available at http://www.unc.edu/~rowlett/units/index.html
- Numericana. http://www.numericana.com
- UK history of measurement http://www.ukmetrication.com/history.htm
- NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. 2011
- NIST Special Publication 447, Weights and Measures Standards of the the United States: a brief history. Lewis V. Judson. 1963; rev. 1976
For many units, the file contains the official definition, so it will be as precise as it gets. The comments often explain where that definition came from.