Mass is the quantity of matter, the unit is kilogram (kg).

The atomic mass is the mass of the naturally occurring mixture of isotopes of an element.

See also Mass Number, Atomic Mass Unit and Isotope.

The mass number is the total number of protons and neutrons (also total nucleon) in the nucleus.

See also Atomic Mass Unit, Atomic Mass.

(AMU) is equal to the mass of one-twelfth of a carbon-12 atom (6 protons + 6 neutrons (+ 6 electrons)), a 'very' little less than 1.66 x 10^-24 gram.

See also Mass Number and Atomic Mass.

[Rest Mass Energy] The term refers to the concept of mass-energy equivalence. Einstein proposed that the equivalence of mass and energy is a general principle.
E=mc²
E = energy,
m = mass,
c = the speed of light in a vacuum.
c² is the conversion factor required to convert from units of mass to units of energy.
The rest energy of an electron or a positron for example amounts to 0.511 MeV.

See also Photon, Photon Energy.