Ordinary treatises define mass like the " the amount of matter in a body " but then we would be able to define first what is matter, and those treatises just define matter in a circular fashion as something that has non-zero mass. Essentially the same problem arises with attempts to define mass using the elusive concept of inertia as " the tendency of a body to keep moving once it is set in motion ". In virtue of the above difficulties, it would be desirable to introduce mass as a primitive concept on the formalism, not defined in terms of anything more fundamental, and associate an operational definition to characterize its measurement. But physicists think different. They introduce mass as a derived concept on relativistic theory. Unfortunately, there is no universal agreement about how to define mass in relativity. The majority of physicists uses a geometric definition of mass, as an Lorentz invariant in spacetime, \(m = \sqrt{E^2 - p^2c^2} / c^2\); a minority