Definition of Chemistry

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].
The first definition captures many of the essential ingredients of chemistry (although definitions 3 and 4 might make a more entertaining paper):

Chemistry is a science. There is only one sanctioned procedure for determining whether a statement about matter is really chemistry: the exhaustive, inefficient, but highly successful scientific method. Chemists often arrive at new results by nonscientific means (like luck or sheer creativity), but their work isn't chemistry unless it can be reproduced and verified scientifically.
Chemistry is a systematic study. Chemists have devised several good methods for solving problems and making observations. For example, analytical chemists often use protocols (thoroughly tested recipes) for determining the concentrations of substances in a sample. Chemists use well-defined techniques like spectroscopy and chromatography to study new or unknown substances.
Chemistry is the study of the composition and properties of matter. Chemistry answers questions like, "What kind of stuff is this sample made of? What does the sample look like on a molecular scale? How does the structure of the material determine its properties? How do the properties of the material change when I increase temperature, or pressure, or some other environmental variable?"
Chemistry is the study of the reactivity of substances. One material can be changed into another by a chemical reaction. A complex substance can by made from simpler ones. Chemical compounds can break down into simpler substances. Fuels burn, food cooks, leaves turn in the fall, cells grow, medicines cure. Chemistry is concerned with the essential processes that make these changes happen.
Chemistry is the study of organic and inorganic substances. Organic substances contain hydrogen combined with carbon; inorganic substances don't. It was once believed that organic compounds were exclusively produced by living things, but today chemists can synthesize many organic materials from inorganic ones. Carbon can link with itself and other atoms in many diverse ways, and its chemistry is far more complex than that of other elements. So while the organic/inorganic distinction is artificial, it's still useful.
Chemistry is the study of connections between the everyday world and the molecular world. Chemists use atoms and molecules to explain properties and behaviors of matter. For example, you can find molecular explanations for flavor and color changes elsewhere on this site.
If you'd like some historical perspective, a good reference is The Enlighment of Matter: The Definition of Chemistry from Agricola to Lavoisier, by Marco Baretta.

Author: Fred Senese senese@antoine.frostburg.edu