Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with mass of an atom of carbon (C-12 isotope) taken as 12 amu

Carbon-12 is considered as unit to calculate atomic mass. Carbon-12 is an isotope of carbon. The relative mass of all atoms are found with respect to C-12.

One atomic mass = 1/12 of the mass of one atom of C-12.

This means atomic mass unit = 1/12th of carbon-12.