One twelfth of the mass of a carbon-12 atom in its nuclear and electronic @G02704@, \(m_{\text{u}} = 1.660\ 5402(10)\times 10^{-27}\ \text{kg}\). It is equal to the @U06554@.
Sources: CODATA Bull. 1986, 63, 1 [Terms] [Book] PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 962 [Terms] [Paper]