noun, plural: unified atomic mass units
One twelfth the mass of a neutral atom of the most abundant isotope of carbon (carbon-12)
The unified atomic mass unit (u) refers to the 1/12 (0.0833) the mass of the carbon-12 atom. It is used to express the mass of atomic and subatomic particles.
It is also called dalton, named after John Dalton. John Dalton proposed in 1803 to use the mass of hydrogen atom as the natural unit for relative atomic mass. The hydrogen was replaced later on with oxygen. Wilhelm Ostwald suggested to base the relative atomic mass in terms of 1/16 mass of oxygen in 1903. Thus, formerly, the atomic mass unit is based on oxygen. Years later (in 1929), isotopic oxygen was discovered and as such it resulted in the divergence in relative atomic mass representation. The divergence could result in errors. To prevent this, carbon-12 was eventually used as the reference standard in 1961.
At this point in time, the atomic mass unit is based on carbon-12. In so doing, it was called unified atomic mass unit. Nevertheless, the term atomic mass unit without the unified is still being used today although it is no longer based on oxygen-16 but on carbon-12.