This paper studies the interactive structure and the interactive meaning of definitions in primary school classroom interaction. The classes that were chosen are classes which consisted solely or for a large part of second language learners, as definitions might have a special importance for them in their second language acquisition. Three categories of definitions will be analyzed: teacher definitions, pupil definitions and jointly formulated definitions. It will be shown that teacher definitions have a very important communicative function. They help the pupils grasp the words and the worlds associated with them. Pupil definitions on the other hand--like most pupil utterances in classroom interaction--often have a completely different communicative function: their main goal is to express the pupil's knowledge. As far as the interactive structure of the definitions is concerned, there appears to be a huge difference between teacher definitions and pupil definitions. Teacher definitions resemble dictionary definitions most. A pupil definition on the other hand is mostly not a one-liner by one person. It often develops into a jointly formulated definition, which is a process of information exchange between teacher and pupil, in the course of which the definition is being built up gradually. (Contains 2 figures and 2 tables.)