Tele-immersive systems development is always driven as well as restricted by the available immersive technology. Hence, existing such systems are described mainly from a technological point of view; their conceptual description is usually limited to the description of a scenario that is implementable with or circumvents the limitations of the chosen technology. This focus on technology makes it difficult to compare systems' concepts; moreover, it has led to different views on tele-immersion in different fields, such as remotely controlled robots, immersive video conferencing, and tele-collaboration. In this work, we give a general, structured principle to describe the conceptual part of any tele-immersion system. This principle naturally unifies the different views on tele-immersion. Our idea is based on the insight that, in order to be general, immersion must be described separately for each direction of communication. We characterize communication between locations using a graph; for each directed edge of this graph, we describe immersion as operations on volumes. Using this principle, we define a typology, which enables the comparison and enumeration of tele-immersion concepts. We apply this typology to survey the concepts of existing tele-immersion systems and thereby demonstrate how three well-known tele-immersive scenarios-Marvin Minsky's tele-operated robot, the Office of the Future, and the asymmetric Beaming scenario-integrate naturally. We show how the general principle can be utilized conveniently to grasp conceptual ideas in tele-immersion, such as direct interaction, locational presence, spatial consistency, symmetries, and self-inclusion.