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(amu,u) A unit of mass equal to 1/12 the mass of a carbon-12 nucleus, which is 1.660 538 73 × 10 -27 kg ± 0.000 000 13 × 10 -27 kg [ 1998 CODATA values ]. Abbreviated as amu or u . Sometimes called the dalton , after John Dalton , architect of the first modern atomic theory .