User tasks play a pivotal role in visualization design and evaluation. However, the term `task´ is used ambiguously within the visualization community. In this article, we critically analyze the relevant literature and systematically compare definitions for `task´ and the usage of related terminology. In doing so, we identify a three-dimensional conceptual space of user tasks in visualization, referred to as task cube, and the more precise concepts `objective´ and `action´ for tasks. We illustrate the usage of the task cube´s dimensions in an objective-driven visualization process, in different scenarios of visualization design and evaluation, and for comparing categorizations ofabstract tasks. Thus, visualization researchers can better formulate their contributions which helps advance visualization as a whole.