Don't get me wrong. I'm not here to ask for any standard textbook answer. This actually came from a teacher of mine who raised this interesting question. All physics textbooks start with the sentence "mass is a measure of matter". But that doesn't really make sense if you think it out clearly. So what kind of quantity is it actually measuring, anyway? I mean, for the same number of atoms, you can get different masses for different elements. Is it the same number of electrons and protons and neutrons and.....well, you get the picture. But no....we say that a electron is approximately ~1/1867(sorry if I got the wrong number) the mass of a proton. Where does THAT come from, anyway? Most people try to say that same masses weigh the same. However, that doesn't make sense either. Weight is actually gravitational force, which Newton proved to be based on mass(F=GM1M2/r^2). So, you can't use such a force to define mass. Actually, you can't even use force, because force is defined as the acceleration of mass. Using them would the same as using mass to define mass. And since I believe that most of us still remember that long winded argument over what a force is, I wouldn't suggest using force to define mass, which is another approach. Some people talk about Newton's first law as a definition of mass: "Some property of matter that opposes acceleration". That has a clear definition of what mass is, and brings out the idea of equality of mass, which is "two pieces of matter that have equal ability to oppose change in motion". But still, there is a critical piece missing in the picture: the quantitization of mass. What is equal mass? What is twice? What is half? If we don't first define force, we don't get anywhere, because different forces give rise to different accelerations....but how can we first have force if it depends on mass? Adding relativity into the picture makes this even more strange, because it kills the idea of "absolute mass" and brings in "relativistic mass". So, perhaps it would be better to stick with classical physics for the while. Time, mass and displacement have probably been the three most critical elements in physics. Time has already been an all time confusing puzzle. Displacement is now getting more and more unclear with the introduction of modern physics. Then comes mass, which doesn't seem to have a good definition at all. None of them had very firm and solid bases before we started working on them. Of course, we could have left all these questions unanswered and continue on with our exploration of "new stuff". But how can we be sure about what we do if we never really knew if we were right in the first place? What do you think?