1. a unit of mass equal to one twelfth of the mass of a carbon-12 atom, used to express the mass of atomic and subatomic particles
Usage: abbreviation for atomic mass unit; also called dalton; used in chemistry and physics

Examples

• The mass of a proton is approximately 1.007 amu.
• Scientists measure the atomic weight of elements in amu.
• One amu is defined relative to the carbon-12 isotope.
• The neutron has a mass of about 1.009 amu.
• Atomic mass units allow chemists to work with very small particle masses.