We detail and illustrate glanceability as a crucial requirement for several types of mobile visualizations. For example, in a difficult terrain, a runner can only check a smartwatch for elevation or heart rate data for a few hundred milliseconds before the eyes need to refocus on the trail ahead. Such quick information needs differ from those in traditional visualizations that are meant for deep analysis and interaction with possibly large and complex datasets. Visualizations designed for quick information needs are described under a variety of terms in the literature such as glanceable visualizations, glanceable displays, peripheral displays, ambient visualizations, or sometimes as forms of casual visualizations. To clarify how glanceability is used in the field of Visualization, we discuss these related terms with respect to visualization concepts, drawing from not only Visualization but also, Vision Sciences, HumanComputer Interaction, and Ubiquitous Computing, revealing how the use of the term glanceable differs in these communities. Drawing from these different perspectives, we discuss specific values for glanceable mobile visualizations: presence & accessibility, simplicity & understandability, as well as suitability & purposefulness. Based on these values, we explore different evaluation methodologies, ranging from lab studies, to online experiments, to evaluation in the field and conclude with a discussion of open challenges in the design of future glanceable mobile visualizations.