The unit used to express weighted average atomic mass is the dalton, also known as the unified atomic mass unit. One dalton is defined as 1/12 of the mass of a carbon-12 atom, which contains six protons and six neutrons..
The name "dalton" was chosen to celebrate the life of John Dalton, an English scientist who proposed an atomic theory of matter in 1803. The unit symbol for the dalton is "Da," following the convention of not capitalizing the name of units based on proper nouns and capitalizing the symbols for those units instead.
The unified atomic mass unit is technically an archaic name for the dalton, but this name continues to be used in many scientific publications due to its familiarity. Since 2003, the name "dalton" has been preferred by the Consultative Committee for Units.