The Unified Atomic Mass Unit (u) or Dalton (Da) is a standardized unit of mass roughly corresponding to that of one Nucleon (i.e., Proton or Neutron), but fixed precisely enough to indicate small differences. It is defined as 1/12 the mass of Carbon (C)-12. and amounts to 1.66053892 x 10-27 kg.
The Dalton is used for specifying the mass of atomic nuclei, atoms, and molecules. 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.
It replaced the earlier Atomic Mass Unit (amu), which is similarly based on Oxygen (O)-16.