In this article the concept of mass is analyzed based on the special and general relativity theories and particle (quantum) physics. The mass of a particle (m=E(0)/c^2) is determined by the minimum (rest) energy to create that particle which is invariant under Lorentz transformations. The mass of a bound particle in the any field is described by m<E80)/c^2 and for free particles in the non-relativistic case the relation m=E/c^2 is valid. This relation is not correct in general, and it is wrong to apply it to the radiation and fields. In atoms or nuclei (i.e. if the energies are quantized) the mass of the particles changes discretely. In non-relativistic cases, mass can be considered as a measure of gravitation and inertia.
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