Answer:
It is a weighted average of the masses of an element’s isotopes.
Explanation:
The best definition of the atom mass of an element is that it is a weighted average of the masses of an element's isotopes.
 Atomic mass  = ∑m[tex]_{i}[/tex] a[tex]_{i}[/tex]
where m[tex]_{i}[/tex] is the mass of the isotope i
      a[tex]_{i}[/tex] is the abundance of the isotope i