Question

define the atomic mass unit in 2-3 lines

Answer 1

An atomic mass unit (symbolized AMU or amu) is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.

Answer 2

a unit of mass used to express atomic and molecular weights, equal to one twelfth of the mass of an atom of carbon-12.

atomic mass unit is equal to the 1/12th mass of carbon-12 atom, it is used to measure mass of atoms and molecules.