The unified atomic mass unit (Also known as amu, symbol: u) or dalton (symbol: Da) is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538921(73)×10⁻²⁷ kg. One dalton is approximately equal to the mass of one proton or one neutron; an equivalence of saying 1 g mol⁻¹ . The CIPM have categorised it as a "non-SI unit" because units values in SI units must be obtained experimentally.

The atomic weight scale has traditionally been a relative scale, that is without an explicit unit, with the first atomic weight basis suggested by John Dalton in 1803 as ¹H. Despite the initial mass of ¹H being used as the natural unit for atomic weight, it was suggested by Wilhelm Ostwald that atomic weights would be best expressed in terms in units of 1/16 weight of oxygen. This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.

The atomic mass (m_a) is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom.

The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight; these differ subtly from the atomic mass. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average as in the case of atomic weight. In the case of many elements that have one dominant isotope the actual numerical similarity/difference between the atomic mass of the most common isotope and the relative atomic mass or standard atomic weights can be very small such that it does not affect most bulk calculations—but such an error can be critical when considering individual atoms. For elements with more than one common isotope the difference even to the most common atomic mass can be half a mass unit or more (e.g. chlorine). The atomic mass of an uncommon isotope can differ from the relative atomic mass or standard atomic weight by several mass units.