What is atomic mass unit?

The atomic mass unit is equal to 1/12^th mass of 1 atom of carbon-12.
One atomic mass unit (amu) is defined as the 1/12th mass of 1 atom of carbon-12, chosen as a reference. Mass units of all other elements are taken relative to 1/12th mass of 1 atom of carbon-12, which is 1 amu. The atomic unit mass is symbolized as amu.

1 amu = Average proton rest mass and the neutron rest mass.
1 amu = 1.67377 x 10^-27 kilogram or 1.67377 x 10^-24 gram.