It is the average relative mass of an atom of the element as compared to the mass of 12 carbon atom taken as 12 units.
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Mass of atom

An atomic mass (symbol: ma) is the mass of a single atom of a chemical element. It includes the masses of the 3 subatomic particles that make up an atom: protons, neutrons and electrons. Atomic mass can be expressed in grams. … 1 atomic mass unit is defined as 1/12 of the mass of a single carbon-12 atom.