One twelfth of the mass of a carbon-12 atom in its nuclear and electronic
ground state
, . It is equal to the
unified atomic mass unit
.
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]