The lack of an agreed upon definition of information system is one of many obstacles troubling the academic information systems discipline. After listing a number of definitions of IS, this paper defines information system as a special case of work system as defined in Alter (1999a). This definition has many desirable characteristics: It is easy to understand; differentiates IS from IT; covers totally manual, partially automated, and totally automated information systems; links to a life cycle model that generates many insights about development and implementation problems; provides a simple guideline that helps in interpreting common IS/IT jargon; and has other useful implications related to IS concepts, IS terminology, and the analysis and design of information systems. The paper presents the proposed IS definition and evaluates the definition in terms of simplicity, clarity, scope, systematic power, explanatory power, validity, reliability, and fruitfulness. An Appendix summarizes previously published concepts and two frameworks that flow from the proposed definition and are useful for appreciating many points in the evaluation section.