- a unit of mass, equal to 1/12 (0.0833) the mass of the carbon-12 atom and used to express the mass of atomic and subatomic particles.
- (formerly) a unit of mass, equal to 1/16 (0.0625) the mass of an oxygen atom having atomic mass 16.Abbreviations: amu, AMU
- a unit of mass used to express atomic and molecular weights that is equal to one twelfth of the mass of an atom of carbon-12. It is equivalent to 1.66 × 10 –27 kg
