This article argues that many key theoretical concepts and core areas of study in the educational sciences are couched in paradigmatically vague terms. The shared features of vague terms and two different readings of vagueness are discussed. "Practice", which is widely used both as a theoretical and an empirical term in the field of educational research, is used as an example of a vague concept that cannot be made more precise, regardless of the quality of definitions or theoretical agreement. Finally, in addition to giving an outline of how the term "practice" is used in the educational sciences, the article refers to contemporary educational research to discuss some of the key methodological implications of vague terms for researchers.