relative atomic mass

Relative atomic mass (symbol: A) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element (from a single given sample or source) to of the mass of an atom of carbon-12 (known as the unified atomic mass unit). The relative atomic mass is a statistical term, referring to an abundance-weighted figure involving measurement of many atoms. As in all related terms, the word "relative" refers to making the figure relative to carbon-12, so that the final figure is dimensionless.