Define one atomic mass unit (a.m.u.)?

A carbon-12 atom's mass is precisely 1/12 of an atomic mass unit. The nucleus of an atom of carbon-12 has six neutrons and six protons. It's written as a.m.u or u. ( unified). The average of the neutron rest mass and the proton rest mass is 1 a.m.u.
1 a.m.u. $=1.67377\times {10}^{-27}$ kilogram or $1.67377\times {10}^{-24}$ gram.