/ Dictionary / Index A atomic mass unit: Meaning and Definition of Find definitions for: atom'ic mass' u"nit Pronunciation: [key] — Physics. Physics. Also calleda unit of mass, equal to 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 the mass of an oxygen atom having atomic mass 16. Abbr.: amu, AMU Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. atomic mass atomic number 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