Based upon selected aspects of the theory of definition as proposed by Leonard (1967), results of the different conceptions of language proficiency are discussed. Properties of definitions, such as ambiguity and vagueness, and basic rules for defining terms are applied to current definitions of language proficiency in order to evaluate their adequacy. Rules of definition include clarity, avoidance of figurative language, avoidance of simple synonyms, and provision of the essential characteristics of the term being defined. English language proficiency has been given different operational definitions depending on the context in which it has been discussed. Guidelines for formulating clearer and more precise definitions of language proficiency are offered. Assessors should specify the attributes with which they are concerned, distinguish between trait names and trait definitions, and consider the types of individuals that they are trying to assess. (RW)