We may have an intuitive understanding of what is meant by mobile data visualization. Yet, in the context of data visualization, the term mobile can be interpreted in several ways. For example, it may describe visual representations shown on devices that are inherently mobile. It may also describe visualizations meant to react to viewers who are mobile relative to the display. Alternatively, it may describe visualizations that are themselves mobile across devices and screens, or in space. In this chapter, we propose several characteristics that help us to identify and describe the scope of mobile data visualization. We focus on the characteristics that, particularly in their extremes, differentiate mobile data visualization from other forms of data visualization. These characteristics give rise to dimensions of a design space for mobile data visualization, against which instances may be classified and positioned. We discuss a number of examples to illustrate how the design space makes it possible to describe and compare mobile visualizations.