All of science (be it physics, chemistry, biology, etc...) aims to study the physical world using the so-called Scientific Method. Physics is the particular not in terms of what general areas of the world it looks at, but more in the techniques used to study it.
Physics is the study of aspects of the world which are amenable to simple mathematical models which can be used to test and verify the hypotheses of the physicist (of course, "simple" is in the eye of the beholder...). If the models get too complex, with too many moving parts, the subject suddenly starts getting called, e.g. in the case of complicated atoms: chemistry.
But when you get right down to it, depending on the broadest view of what constitutes a "simple mathematical model", physics is the scientific study of our world. All of it.