atomic mass unit

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

• noun A unit of mass equal to ¹/₁₂ the mass of the most abundant isotope of carbon, carbon-12, which is assigned a mass of 12.

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

• noun chemistry A unit of mass equal to 1/12 the mass of an atom of the 12C isotope of carbon.

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

• noun unit of mass for expressing masses of atoms or molecules