The process of developing definitions is underemphasized in most mathematics instruction. Investing time in constructing meaning is well worth the return in terms of the knowledge it imparts. In this article, the authors present a third approach to "defining," called "constructive." It involves modifying students' previous understanding of a term and creating a new idea. The authors show that using "constructive defining" opened up opportunities for students to create more meaningful strategies for finding their own formulas for the sum of the interior angles of a polygon. (Contains 7 figures and 1 table.)