And I want to be careful with this definition right here because there are other things that aren't, what would be what we traditionally associate with matter and I don't want to start going into more fancy physics that still will exhibit mass, so another way to define mass is how does something react to a specific force, we have already learnt it in the second law. if you have a given force, and you have more mass you'll accelerate less and if you have less mass, you'll accelerate more.