atomic mass unit
The atomic mass unit is the standard unit in which the masses of atoms and molecules are measured. One atomic mass unit (amu) is defined as one-twelfth of the mass of a single atom of the isotope carbon-12. This is 1.6605 × 10-27 kg, or approximately 931 MeV.
The above definition was agreed upon by the International Union of Pure and Applied Physics in 1960 and the International Union of Pure and Applied Chemistry (in 1961), resolving a long-standing difference between chemists and physicists. The current atomic mass unit, also called the unified atomic mass unit, replaced the atomic mass unit (chemical scale) and the atomic mass unit (physical scale), both having the symbol amu. The amu (physical scale) was one-sixteenth of the mass of an atom of oxygen-16. The amu (chemical scale) was one-sixteenth of the average mass of oxygen atoms as found in nature. 1 amu (unified) = 1.000 317 9 amu (physical scale) = 1.000 043 amu (chemical scale).