This article examines the coordination of speech and gesture in teachers' definition talk, that is, vocabulary explanations addressed to language learners. By analyzing one ESL teacher's spoken definitions, the study demonstrates in the details of the unfolding talk how a teacher crafts and choreographs his definitions moment by moment, while attending to the student's linguistic needs. The study draws upon McNeill's (1999/1992) notion of "gesture-speech co-expressiveness," McNeill and Levy's (1993) idea of "gestural cohesion," and Kendon's (2000) notion of "turn contextualization through gesture." The study identifies three major functional roles of gesture: reinforcing the meaning of verbal utterances; disambiguating the meaning of lexical items; and establishing gestural cohesion across turns at talk. Through the fine-grained analysis of five excerpts, this study contributes to our understanding of the interactional and informational aspects of gesture in second language teaching. The article discusses implications for the study of gesture in general, and the role of gesticulations in providing comprehensible input to second language learners in particular. (Contains 9 figures, 3 excerpts, and 10 notes.)