An atomic mass unit (AMU) is a unit of mass used to express the mass of atomic and subatomic particles. It is defined as one-twelfth of the mass of an atom of carbon-12 (12C), which is the most abundant isotope of carbon found in nature, and has an AMU of 12/04%3A_Atomic_Structure/4.19%3A_Atomic_Mass_Unit). The AMU is also known as the Dalton (Da) or unified atomic mass unit (u) ). The unified atomic mass unit is generally considered a synonym for AMU and is a physical constant accepted for use in the International System of Units (SI) measurement system.

The AMU is used to express both atomic masses and molecular masses. It is a useful way to differentiate between isotopes by expressing their relative masses. An isotope refers to multiple elements with the same atomic number but a different atomic mass due to a different number of neutrons.

In summary, an AMU is a unit of mass used to express the mass of atomic and subatomic particles. It is defined as one-twelfth of the mass of an atom of carbon-12 and is also known as the Dalton or unified atomic mass unit.