Abstract: In most fluid dynamics applications, unsteady flow is a natural phenomenon and steady models are just simplifications of the real situation. Since computing power increases, the number and complexity of unsteady flow simulations grows, too. Besides time-dependent features, scientists and engineers are essentially looking for a description of the overall flow behavior, usually with respect to the requirements of their application domain. We call such a description a flow structure, requiring a framework of definitions for an unsteady flow structure. In this article, we present such a framework based on pathline predicates. Using the common computer science definition, a predicate is a Boolean function, and a pathline predicate is a Boolean function on pathlines that decides if a pathline has a property of interest to the user. We will show that any suitable set of pathline predicates can be interpreted as an unsteady flow structure definition. The visualization of the resulting unsteady flow structure provides a visual description of overall flow behavior with respect to the user's interest. Furthermore, this flow structure serves as a basis for pathline placements tailored to the requirements of the application.