Why do definitions play such a central role in mathematics? It may seem obvious that precision about the terms one uses is necessary in order to use those terms reasonably (while reasoning). Definitions are chosen so as to be definite about the terms one uses, but also to make both the statement of, and the reasoning to justify, theorems as elegantly simple and hence more memorable as possible. Engaging learners in the implications of different variations of a definition may lead to an appreciation for learners of the role of definitions, and the reasoning that those definitions enable. In this article, the author explores the central role of definitions in mathematics. The examples appended are intended to display classroom situations where work can be done on refining definitions through construction of examples and counter-examples. (Contains 1 figure and 1 footnote.)