1、The relative atomic mass is the weighted average mass of naturally occurring atoms of an element on a scale where an atom of carbon-12 has a mass of exactly 12 units.

2、Relative atomic mass, Ar, is defined as the mass of one atom of an element relative to 1/12 of the mass of an atom of carbon-12, which has a mass of 12.00 atomic mass units.

常规教学中通常更喜欢使用后者，这一定义与普通高中化学教材相一致，更好理解。