atomic mass unit

WordReference Random House Unabridged Dictionary of American English © 2026

atom′ic mass′ u′nit, [Physics.]

1. Physics Also called dalton. a unit of mass, equal to ¹⁄₁₂ the mass of the carbon-12 atom and used to express the mass of atomic and subatomic particles.
2. Physics (formerly) a unit of mass, equal to ¹⁄₁₆ the mass of an oxygen atom having atomic mass 16. Abbr.: amu, AMU

• 1950–55

Collins Concise English Dictionary © HarperCollins Publishers::

atomic mass unit noun

1. a unit of mass used to express atomic and molecular weights that is equal to one twelfth of the mass of an atom of carbon-12. It is equivalent to 1.66 × 10^–27 kg
   Abbreviation: amu