## Wikipedia - Atomo unified atomic mass unit

https://doi.org/10.1351/goldbook.U06554
Non-SI unit of mass (equal to the @A00497@), defined as one twelfth of the mass of a carbon-12 atom in its @G02704@ and used to express masses of atomic particles, $$\text{u}\approx 1.660\ 5402(10)\times 10^{-27}\ \text{kg}$$.
Sources:
Green Book, 2nd ed., p. 75 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 997 [Terms] [Paper]