What is atomic mass?

The atomic mass (m_a or m) is the quantity of matter contained in an atom of an element. 

The SI unit of mass is kilogram (kg), but it is often expressed in the non-SI unit, dalton (Da, or u) where 1 dalton is defined as $\frac{1}{12}$ of the mass of a single carbon-12 atom, at rest.

The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number.