Formal definitions are one example of "decontextualized" language use, in which reliance on background knowledge shared with the interlocutor is minimized, and use of conversational devices is avoided. Definitions of English nouns by 137 second- to fifth-grade children, about half of whom were non-native English speakers, were analyzed to assess the children's tendency to use the formal definitional genre, their sophistication in using it, and their tendency to rely on conversational devices during the testing session. Seventy of the children were also tested in French, a second language for most of them. Both the tendency to use formal definitions and the sophistication of the definitions offered were found to increase from second to fifth grade, to be affected by proficiency in the language being tested, and to be related to school achievement as reflected in standardized test scores. Use of conversational devices during the testing session was negatively related to achievement and to language proficiency. The developments in sophistication of formal definitions observed and the relationships to school achievement found suggest that performing well on the task of giving formal definitions requires skill in the use of decontextualized language, that such skill is independent of lexical or syntactic knowledge, and that the decontextualized skills may be crucial to success in classroom discourse and in literacy. (Author/MSE)