Confusion, misinterpretations, and conflicting theoretical beliefs seem to be characteristic of language assessment practices. Much of the confusion and misrepresentation are due to the great variety of terms used in the language assessment process and to incomplete definitions of these terms and their operations. In an attempt to clarify the situation and sort out definitions of language proficiency, properties of definitions, such as ambiguity and vagueness, and basic rules for defining terms are applied to judgments of current definitions of language proficiency. Several current definitions of language proficiency are analyzed and different theoretical viewpoints attached to its meaning are revealed. It could be said that language proficiency is given different operational definitions according to the context in which it is used. Some guidelines to be employed in constructing a definition of the term are suggested after a clarification of the term assessment in relation to measurement and evaluation. Specifically, these guidelines attempt to define the property with which one is concerned in assessment of language proficiency, to distinguish between "trait names" and "trait definitions," and to define variables in connection with the particular types of individuals involved in any situation. (AMH)