Atomic mass is the mass of an atom as measured in atomic mass units. Although standards for calculating the mass of atoms were used in the past, currently scientists use the atomic mass unit. It is the average relative mass of an atom of the element as compared to the mass of C-12 isotope taken as 12 atomic mass units. In fact, atomic mass tells how may times an atom is heavier than 12th part of C-12 isotope.
Atomic mass = Atomicity $\times$ Atomic mass