Define relative atomic mass.

Explanation

Ralative atomic mass of an element is the ratio between the average mass of its isotropes to `1//12^(th)` part of the mass of a carbon - 12 atom. It is denoted as Ar.
Relative atomic mass = `("Average mass of the isotropes of the element")/(1//12^(th) "of the mass of one Carbon-12 atom")`.