In 2008, articles by Bell and Schroeder provided an initial platform from which to develop a coherent definition of the term 'virtual worlds'. Yet over the past ten years, there has been little development of the term. Instead there is confusion in the literature, with the introduction of new terms which are at times used to classify the type of virtual world and at others are used synonymously with the term. At the same time there has been a resurgence of interest in the potential of virtual reality which further muddies the conceptual waters. While the lack of a clear and common understanding of a term is not uncommon, there are implications for researchers and practitioners. To address these issues, this paper presents a new framework for the definition of virtual worlds, arguing what it is for a world to be virtual, the user experience that is a necessary part of this and the technical features which afford this. For the first time the relationships between commonly confused terms and technologies are identified to provide a much needed conceptual clarity for researchers and educators.