atom'ic mass' u"nit
1. 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. (formerly) a unit of mass, equal to 1/16 the mass of an oxygen atom having atomic mass 16. Abbr.: amu, AMU
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.