One twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state, mu = 1.660540210 x 10E-27 kg. It is equal to the unified atomic mass unit.
Sources:
CODATA Bull. 1986, 63, 1 (https://doi.org/10.6028/jres.092.010)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 962 (https://doi.org/10.1351/pac199668040957)