Atomic mass unit
The unified atomic mass unit (Also known as amu) (symbol: u) or dalton (symbol: Da) is a unit that is used for indicating mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....

 in its nuclear and electronic ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...

, and has a value of . One dalton is approximately equal to the mass of one proton or one neutron.