Rest mass of an atom in its ground state. The commonly used unit is the unified atomic mass unit.
Green Book, 2nd ed., p. 20 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
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)