I will preface my question with the fact that I am a high school student with only a general knowledge of physics, though I have learned about a variety of phenomena at the limited level of depth that I am capable of. I am curious about several concepts and observed phenomena that are fundamentally important to our understanding of the universe, but seem to be nebulously defined. Among these terms, 'matter' is of the greatest concern to me. I have seen it defined, variously, as the amount of particles, mass, or even 'stuff' in an object. 'Stuff', obviously, is insufficiently clear to be used. Particles, while more specific, appears fundamentally flawed as the basis for matter, because particles themselves are generally considered to have matter (Particles meaning elementary particles). So only mass is left. Why, then, do we use 'matter' when we already have a more-clearly defined term, mass? If my understanding of these terms is incorrect, and this is the cause of my confusion, please let me know as well.