The area of investigation known as general systems theory or research features the study of systems as interesting in its own right or one fruitful approach to the study of science in general. This leads to an interesting and still open problem, namely, explicating the concept of a system that seems to unify the interests of researchers in this area. Contrary to received opinion, I argue that there is a unique and interesting concept of a system that underlies the expressed (...) interests of general systems researchers and that it can be given a satisfactory explication. The concept in question has systems as extralinguistic entities that we theorize about, namely, certain sorts of sets of re-related elements. Criteria for its explication are suggested; examples of systems, earlier analyses, and various objections are all considered in leading up to a partial formal explication of the concept and a statement of additional open problems. (shrink)