Educators look for "teaching opportunities" within the curriculum to "bring the practice of knowing mathematics in school closer to what it means to know within the discipline". The need to emphasise disciplinarity--and the concomitant canons of logic, consistency, and connections--in the teaching of mathematics is in line with the proficiency strand of "understanding" in the Australian curriculum. One of the key constructs within the mathematics discipline is that of "definition." This article intends first to explain the role of definitions as it is used within the mathematics community; second, the authors illustrate, using two examples, how the disciplinarity surrounding "definitions" can be introduced in secondary mathematics classrooms. (Contains 1 figure.)