Unified atomic mass unit
Symbol: u
The precise definition is that the atomic mass unit (u) is one twelfth of the mass of an isolated atom of carbon-12 (12C) at rest and in its ground state. It is the approximate mass of a hydrogen atom, a proton, or a neutron.
Francis Aston, inventor of the mass spectrometer, used 1/16 of the mass of one atom of oxygen-16 as his unit, which was the definition until 1961.
1u = 1.660538782 (83) * 10-27kg
1kg = 6.022141794216764038 * 1026u