Meaning of atomic weight

atom'ic weight'

Pronunciation: [key]
— Chem. Chem.
  1. the average weight of an atom of an element, formerly based on the weight of one hydrogen atom taken as a unit or on the weight of an oxygen atom, but after 1961 based on the weight of the carbon-12 atom. Abbr.: at. wt.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.