The definition is an important language form in the register of mathematics. Students need to understand the structure of a definition so that they can make sense of the definitions they encounter and so that they can construct their own definitions as part of organising their thoughts about the concepts they have explored. This article suggests that there are many mathematical concepts for which the use of a single formal definition is not helpful in developing students' understanding of the concepts. Rather, more might be gained by having students use language, both mathematical and everyday, in some of the other language tasks that have been suggested.