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The unified atomic mass unit (u, often shortened to atomic mass unit) or dalton (Da) is a standardized unit of mass roughly corresponding to that of a nucleon (i.e., proton or neutron), but fixed precisely enough to allow measurement of smaller differences. It is defined as 1/12 the mass of atomic carbon-12 and its relation to other mass units is termed the atomic mass constant (mu), which is 1.66053906660 × 10-27 kg (and subject to future improved determinations).
The dalton is used for specifying the mass of atomic nuclei, atoms, and molecules (atomic mass). Its precise definition allows fractions to be used to specify mass differences due to the binding energy that binds nucleons together, or to specify average masses of the atoms in a mixture of isotopes.
Until the 1960s, the atomic mass unit was specified as 1/16 the mass of an oxygen atom, but there was not agreement whether to base it upon a particular isotope (versus whether to base it upon Earth abundances). The effort to unify its definition also chose to base it on carbon.