One controversial aspect found in the literature on intelligibility is the lack of consensus among the terminologies and concepts proposed by several scholars. This article presents a state-of-art review of the terminologies and definitions proposed for intelligibility from 1950 to 2003. The terminologies and definitions are presented and discussed, and a comparison among them, with specifications of the similarities found, is shown. At the end, ways of collecting data for further investigations on language learners' intelligibility are recommended.