What is chemistry considered the study of?
While the answers so far are not wrong, I'd like to add a refinement. Chemistry is the study of matter at the level of electron interaction. The basic question in chemistry is about chemical bonding, and as far as we know this is a function of electron interaction. In contrast, radioactivity is also about matter, but is not at all about electron interaction and is not generally considered chemistry (it's considered a part of physics). Also, the chemistry of isotopes (i.e. differing neutron count) is not generally different across different isotopes. This is good, because we need solvents with are deuterium based ($\ce{D=^2H}$) as opposed to hydrogen based for NMR spectroscopy. If we go lower than electrons, we are into quantum mechanics, which is again physics. So electron interaction is a good level at which to characterize chemistry.

EDIT: I realize the original question was for a one-word answer. Frankly, that's not the greatest question, but it's generating answers that make it more interesting, and it's why I answered.

Let me give a counterexample to my own answer to show the difficulty: gases. We learn a lot about gases in chemistry which has nothing to do with chemical bonding; e.g. all the gas equations. I'd argue that most of this is not really chemistry, it's physics. Understanding how temperature, pressure, and volume interact dynamically is a very physical sort of study, not all that different from studying light, or magnetism, or force.  So why do we study gas physics in chemistry? I suspect it's because we use gas physics a lot in chemistry! You need to understand the gas laws to do any real experimentation involving actual gas chemistry; i.e. how would you measure gas as a reactant or product without knowing the gas laws?

So to be clear, if you want one word, I agree with "matter". But I think if you want to get at what chemistry is about, it's not so simple.