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For over a century the definitions of mass and derivations of its relationwith energy continue to be elaborated, demonstrating that the concept of massis still not satisfactorily understood. The aim of this study is to show that,starting from the properties of Minkowski spacetime and from the principle ofleast action, energy expresses the property of inertia of a body. This impliesthat inertial mass can only be the object of a definition - the so calledmass-energy relation - aimed at measuring energy in different units, moresuitable to describe the huge amount of it enclosed in what we call the"rest-energy" of a body. Likewise, the concept of gravitational mass becomesunnecessary, being replaceable by energy, thus making the weak equivalenceprinciple intrinsically verified. In dealing with mass, a new unit ofmeasurement is foretold for it, which relies on the de Broglie frequency ofatoms, the value of which can today be measured with an accuracy of a few partsin 10^9.

A re-interpretation of the concept of mass and of the relativistic mass-energy relation