Just off the top of my head I would say the study of atoms, molecules, and subatomic particles on a system and how they react to surroundings and eachother. Physics and chemistry have a lot of overlap.
The dictionary definition is as follows:
"That branch of science which treats of the composition of substances, and of the changes which they undergo in consequence of alterations in the constitution of the molecules, which depend upon variations of the number, kind, or mode of arrangement, of the constituent atoms. These atoms are not assumed to be indivisible, but merely the finest grade of subdivision hitherto attained. Chemistry deals with the changes in the composition and constitution of molecules."