Which isotope defines the unified atomic mass unit?

The isotope that defines the unified atomic mass unit is carbon-12.

One unified atomic mass unit, also called one dalton and written u or Da, is defined as exactly one-twelfth of the mass of an unbound, neutral carbon-12 atom in its ground state. Consequently, a carbon-12 atom has an atomic mass of exactly 12 daltons by definition.

Carbon-12 contains six protons and six neutrons and is the most abundant stable carbon isotope. Scientists use it as the reference point for comparing atomic and molecular masses, so values such as oxygen-16’s mass are expressed relative to carbon-12 rather than measured as unrelated absolute numbers.

Oxygen once served as a mass reference in different forms for physicists and chemists, creating a split standard. Carbon-12 replaced those conventions in 1961. Do not confuse this definition with carbon-14 dating: carbon-14 is radioactive and is useful for age estimation, but it does not define the atomic mass unit.