We analyze the process of constructing a definition within the theoretical framework of "Abstraction in Context." Pairs of students were engaged in a task designed to engender a need for a definition of a tangent to a graph at a given point, and lead to constructing a definition following that need. The results of our analysis point to two characteristics of the process of constructing a definition: a. Constructing the concept does not necessarily include constructing its definition; in particular, students were able to use the concept before constructing its definition. b. Students' language becomes more precise during the constructing process, not only as a characteristic of the process but also as a means of promoting the process.