It has been said that mass is a measurement of inertia.
We can illustrate this mathematically using force, momentum, and even energy. Momentum describes how impacts and collisions will affect other objects. The equation for momentum is p=mv, where p is the momentum of an object, m is mass [a measurement of the inertia] and v is velocity. If two objects collide at equal speed, but one is twice as massive, then the two objects will move in the direction of the more massive object, at a slower speed. You can try this with marbles and a table.
Force is defined as the rate of change of momentum, or `F=d/[dt]p` . This means that mass is changing how forces affect objects. It will take a greater force, for example, to move a bowling ball than a feather.
Energy is the integral of force, or `E=int_()Fdx` . This nest egg of formulas means that once again, mass is playing a role. Here, if two objects are moving with equal velocity, but have different masses, the one with greater mass will have greater energy. Think of how it feels to get hit by an RC car at five miles an hour versus a 200 pound linebacker at five miles an hour.
Based on all of this, it would seem that the mass of an object, in any unit, is really just a quantity of inertia.