The atomic mass unit (AMU or amu) is a unit of mass used to describe matter at the atomic scale/04%3A_Atomic_Structure/4.19%3A_Atomic_Mass_Unit). It is defined as one-twelfth the mass of an atom of carbon-12 (12C), which is the most abundant natural carbon isotope, accounting for over 98% of carbon found in nature. The AMU is also known as the dalton (Da) or unified atomic mass unit (u), and it expresses both atomic masses and molecular masses. The unified atomic mass unit is generally considered a synonym for AMU, and it is a physical constant accepted for use in the International System of Units (SI) measurement system. The relationship between the unified AMU and the SI unit for mass (kg) is expressed by Avogadros number NA, and by the definition of NA, the mass of a 12C atom at rest and in its ground state is 12 grams or 0.012 kg.

The AMU is a useful way to differentiate between isotopes by expressing their relative masses. An isotope refers to multiple elements with the same atomic number (number of protons) but a different atomic mass due to a different number of neutrons. The AMU is used to describe an elements atomic weight, which is the weighted average of the atomic masses of an elements naturally occurring isotopes.

In summary, the AMU is a unit of mass used to describe matter at the atomic scale, and it is defined as one-twelfth the mass of an atom of carbon-12. It is used to differentiate between isotopes and to describe an elements atomic weight.