Atomic mass units (amu) and 1/12th of the mass of carbon-12 are related concepts, but there is a slight difference between them.
Atomic mass units (amu) are a unit of mass used to express the relative masses of atoms and molecules. The atomic mass unit is defined as 1/12th the mass of an atom of carbon-12. It is a convenient unit to compare the masses of different atoms, where 1 amu is approximately equal to the mass of a proton or a neutron.
On the other hand, 1/12th of the mass of carbon-12 is a specific value used as the basis for atomic mass units. Carbon-12 isotope is defined to have a mass of exactly 12 atomic mass units. Therefore, 1 amu is defined as 1/12th the mass of a carbon-12 atom.
In summary, atomic mass units (amu) are a unit of mass, and 1/12th of the mass of carbon-12 is the specific value used to define the atomic mass unit. So while they are related, the term "atomic mass unit" is a broader concept encompassing the unit of mass, while "1/12th of the mass of carbon-12" refers to the specific value used in defining that unit.