As the number of moving objects increases, the challenges for achieving operational goals w.r.t. the mobility in many domains that are critical to economy and safety emerge dramatically. This, in domains such as the Air Traffic Management domain, dictates a shift of operations' paradigm from location-based, as it is today, to trajectory-based, where trajectories are turned into "first-class citizens". Additionally, the increasing amount of data from heterogenous and disparate data sources, imply the need for advanced analysis methods that require exploiting spatio-temporal mobility data in appropriate forms and at varying levels of abstraction. All these call for an in-principle way for organising integrated views of mobility data, with trajectories playing the main role. In this paper, based on a comprehensive framework identifying fundamental spatio-temporal data types and specific conversions among these types, we propose an ontology for modelling semantic trajectories, integrating spatio-temporal information regarding mobility of objects, at multiple, interlinked levels of abstraction, as needed by analysis methods. We validate the ontological specifications towards satisfying the needs of visual analysis tasks in the complex Air Traffic Management domain, using real-world data. 1 Introduction Many tasks in critical domains w.r.t. economy and safety, such as Flow and Traffic Management in the aviation domain, impose emergent and challenging problems, re