Overview is a frequently used notion and design goal in information-visualization research and practice. However, it is difficult to find a consensus on what an overview is and to appreciate its relation to how users understand and navigate an information space. We review papers that use the notion of overview and develop a model. The model highlights the awareness that makes up an overview, the process by which users acquire it, the usefulness of overviews, and the role of user-interface components in developing an overview. We discuss the model in relation to classic readings in information visualization and use it to generate recommendations for future research.