This contribution proposes a definition and taxonomy of the often used term cross-device interaction. Despite of technical progress, systems and interfaces that integrate into the environment are still the subject of intensive research. We still live in a world where devices reside in the foreground and present themselves and their interaction capabilities to the user. At the same time, computing devices become an integral part of our environment, be it in the form of public displays or mobile computers. Furthermore, the number of devices a user owns or has access to is increasing. Humans interacting consciously with multiple devices can be seen as an intermediate stage towards ambient environments or ubiquitous computing. The term cross-device interaction (XDI) is often used to refer to the underlying interaction paradigm in such environments. Unfortunately, the term still lacks consistent and concise definitions. This can be a problem as different authors use the term XDI with divergent meanings within a wide variety of application contexts. To mitigate this problem, we propose a taxonomy and give a user-, space- and interaction-centric definition for XDI. Additionally, we make use of this taxonomy to classify XDI-scenarios found in the literature and the concepts of XDI they exhibit. Keywords-human computer interaction; computer interfaces; context awareness; collaborative work; ambient computing.