Visualization is an inherently interdisciplinary research area: visualization researchers are always visualizing other people's data. While recent trends in the field has turned toward a more inclusive audience, particularly within the topic of “visualization for the masses”, the traditional user group for our tools have been the domain expert: people who are experts in a specific professional domain where they want to apply visualization and analytics, but who often lack high literacy, training, and motivation in visualization and visual analytics. Such domain experts want to opportunistically reap the benefits of visualization, but have no patience for long training, poor interaction or visual design, or complex displays. While domain experts are a familiar user group, surprisingly little effort has been devoted towards characterizing them for both design and evaluation purposes. To help the visualization community better understand this specific group, in this paper, we describe the characteristics of domain experts, discuss existing examples designed for them, and propose possible guidelines to facilitate the design process. We believe this discussion will help visualization researchers better understand this group and uncover more research opportunities.