Any physical quantity is a measure of a property of a physical system.We are concerned with the property inertia. It is known to be measured by thephysical quantity mass. We were concerned with the question of whether massis a measure of the property inertia. For this purpose, we first settled what wewant to understand by inertia, and we introduced a measure for it. As long asone considers movements with center-of-mass velocities much smaller thanthe terminal speed c, relativistic mass measures inertia. In general, however, itdoes not. If we allow for relativistic center-of-mass velocities, the mass loses thisproperty. Neither the rest mass nor the relativistic mass is a measure of inertia.However, the inertial behavior can now be described by a characteristic curve.