SUMMARYIn the literature on linguistics, language acquisition, and disordered language there is much confusion in the use of terminology (Lyons, 1970; Sheridan, 1972; Crystal, 1980, 1982a). Even basic terms which have quite distinct meanings are often used interchangeably, seriously hindering staff whose job it is to meet the needs of people who have problems with language. As Crystal (1976) states, terms should be clearly defined and consistently used.This article explores the meanings of certain key terms in the hope that they will be better understood, and more consistently used, by staff working with mentally handicapped people. It also indicates some important aspects of language, paving the way for a discussion of the language curriculum in subsequent articles.