The $\textit{Atomic Mass Constant}$ is one twelfth of the mass of an unbound atom of carbon-12 at rest
and in its ground state.
http://physics.nist.gov/cuu/Constants/bibliography.html
http://en.wikipedia.org/wiki/Atomic_mass_constant
http://dbpedia.org/resource/Atomic_mass_constant
http://www.iso.org/iso/catalogue_detail?csnumber=31895