atomic mass constant
One twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state, Math - ei. It is equal to the unified atomic mass unit
Source:
CODATA Bull. 1986, 63, 1
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 962