There isn't one single "best" definition of chemistry, as it's a vast and complex field. But here are a few options, each highlighting a different aspect:

1. Focus on the study of matter:

* Chemistry is the study of matter and its properties as well as how matter changes. This definition emphasizes the core subject of chemistry: the stuff that makes up the universe.

2. Focus on the interactions between matter:

* Chemistry is the study of the composition, structure, properties, and reactions of matter. This definition expands on the first by including how matter interacts with itself and its environment.

3. Focus on the fundamental building blocks:

* Chemistry is the study of atoms and molecules and their interactions. This definition highlights the core building blocks of matter and their behavior.

4. Focus on the application of knowledge:

* Chemistry is the science that deals with the identification, composition, structure, properties, and reactions of matter. This definition emphasizes the practical applications of chemical knowledge.

Ultimately, the best definition for you will depend on your specific needs and the context in which you are discussing chemistry. Each definition captures a different aspect of this fascinating and vital field.