unified atomic mass unit

https://doi.org/10.1351/goldbook.U06554
Non-SI unit of mass (equal to the atomic mass constant), defined as one twelfth of the mass of a carbon-12 atom in its ground state and used to express masses of atomic particles, u ≈ 1.660 5402 10 x 10E-27 kg.
Sources:
Green Book, 2nd ed., p. 75 (https://dev.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 997 (https://doi.org/10.1351/pac199668040957)