Dictionary
atom'ic mass'
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
1/16 the mass of one oxygen atom, but after 1961 based on
1/12 the mass of the carbon-12 atom.
Abbr.: at. m.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.