ANSWER:
Relative Atomic Mass:
“Relative atomic mass is the mass of an atom of an element as compared to the mass of an atom of carbon taken as 12.”
We use the term relative atomic mass because the given masses of atoms are not their absolute masses. Rather, they are the relative masses because they are calculated by comparing with the mass one atom of carbon, arbitrarily taken as 12.0000 amu.
Examples: The relative atomic masses of H and O are 1.008 amu and 15.9995 amu, respectively.
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 14)