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Meaning of atomic mass

atom'ic mass'

Pronunciation: [key]
— Chem. Chem.
  1. 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 the mass of one oxygen atom, but after 1961 based on the mass of the carbon-12 atom. Abbr.: at. m.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.