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The Atomic Mass Unit (symbol: u), also known as the Dalton (symbol: Da), is a standard unit of mass used to express the masses of atoms and molecules. It is defined as exactly one-twelfth (1/12) of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. This definition provides a convenient scale for comparing the masses of different isotopes and molecules.

The concept of a relative atomic mass scale emerged in the early 19th century with John Dalton. Initially, hydrogen was used as the standard. Later, oxygen became the standard (first as natural oxygen, then oxygen-16). In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the current standard based on carbon-12 to resolve discrepancies between the physics and chemistry scales that used oxygen-16 and natural oxygen, respectively. The name "Dalton" (Da) was proposed later and is now widely accepted, especially in biochemistry and molecular biology.

• Chemistry: Expressing atomic weights of elements and molecular weights of compounds. Essential for stoichiometry calculations.
• Physics: Measuring the masses of subatomic particles, nuclei, and atoms with high precision. Used in mass spectrometry.
• Biochemistry & Molecular Biology: Expressing the masses of large molecules like proteins and nucleic acids, often using the kilodalton (kDa) or megadalton (MDa).