"chem·is·try n., pl. -tries. 1. the science that systematically studies the composition, properties, and activity of organic and inorganic substances and various elementary forms of matter. 2. chemical properties, reactions, phenomena, etc.: the chemistry of carbon. 3. a. sympathetic understanding; rapport. b. sexual attraction. 4. the constituent elements of something; the chemistry of love. [1560-1600; earlier chymistry]."
My glossary definition is short and sweet: Chemistry is the "scientific study of matter, its properties, and interactions with other matter and with energy".
An important point to remember is that chemistry is a science, which means its procedures are systematic and reproducible and its hypotheses are tested using the scientific method. Chemists, scientists who study chemistry, examine the properties and composition of matter and the interactions between substances. Chemistry is closely related to physics and to biology. Chemistry and physics both are physical sciences. In fact, some texts define chemistry and physics exactly the same way. As is true for other sciences, mathematics is an essential tool for the study of chemistry.