Simple the definition lies in the meaning itself. The mass of an atom is refereed as its atomic mass.
Measured in:- amu or atomic mass unit
Commonly the atomic mass is calculated by adding the number of protons and neutrons together whereas electrons are ignored.
Expressed in:- grams or any other units to measure weight.
Standard:- 1/12th of mass of a C-12 isotope.
In case of H, He , LI , Be , B , C , N , O , F , Ne , Na , Mg , Al , Si , P, S , Cl, Ar , K , and Ca except Li , Be, B, F, Na , Al , P, Cl , K and Ar,
atomic mass= double the atomic number
Here are some of the elements and their atomic mass-