atomic weight

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. Former term for the more specific relative atomic mass.
2. n. 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.

GNU Webster's 1913

1. adj. (Chem.) the weight of the atom of an element as compared with the weight of the atom of hydrogen, taken as a standard.
2. n. (Chem.) See under Atomic, and cf. Element.

WordNet 3.0

1. n. (chemistry) the mass of an atom of a chemical element expressed in atomic mass units