| the mass of an isotope of an element measured in units formerly based on the mass of one hydrogen atom taken as a unit or on 1/16 the mass of one oxygen atom, but after 1961 based on 1/12 the mass of the carbon-12 atom. Abbreviation: at. m. |

atomic mass n.
The mass of an atom, usually expressed in atomic mass units.
| atomic mass
The mass of a given atom or molecule, expressed in atomic mass units. Compare atomic weight. See also mass number. |