/ Dictionary / Index A atomic mass: Meaning and Definition of Find definitions for: atom'ic mass' Pronunciation: [key] — Chem. Chem. 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. atomicity atomic mass unit Related Content Daily Word Quiz: crutiny Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language