The use of Requirements at Runtime (RRT) is an emerging research area. Many methodologies and frameworks that make use of requirements models during the execution of software can be found in the literature, but very few of them use ontologies to ground the models that are used at runtime. In this paper, we introduce the Runtime Requirements Ontology (RRO), a domain ontology that intends to represent the nature and context of RRT. Following a well-known Ontology Engineering method, we evaluate RRO using verification and validation techniques.