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The concept of “relevance”, sometimes also called “pertinence” or “aboutness”, is central to the theory of information retrieval. Unfortunately, however, there is at present no consensus as to how this notion should be defined. The purpose of this paper is to propose and defend a definition of what it means to say that a piece of stored information is “relevant” to the information need of a retrieval system user. The suggested definition explicates relevance in terms of logical implication. For any yes-or-no question answering system which operates with one of the standard formalized languages, the definition provides a mathematically precise criterion of relevance. For other types of fact retrieval systems and reference retrieval systems, including all systems whose stored information is expressed in natural language, the definition is not mathematically precise but is nevertheless still helpful on a conceptual level.