atomic weight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The average mass of an atom of an element, usually expressed relative to the mass of carbon 12, which is assigned 12 atomic mass units.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Former term for the more specific relative atomic mass.
• noun A term used to represent the mean relative atomic mass of an element in nature, as distinct from the relative atomic mass of a single isotope.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (chemistry) the mass of an atom of a chemical element expressed in atomic mass units