How Can Geometry Students Understand What It Means to Define in Mathematics?

This article discusses how a teacher can prepare the terrain for students to understand what it means to define a figure. Drawing on writings from mathematicians and mathematics educators on the role of definitions in mathematics, the authors argue that students develop a greater appreciation for the conciseness of a mathematical definition if they are involved in activities of generating figures that meet stipulated properties. The authors illustrate that argument with episodes from students’ play of a game called Guess My Quadrilateral! in two high school geometry classes.