Goal-Oriented Requirements Engineering (GORE) has grown into an important area of research in the past decades. Still, some of its corners remain dark, since different GORE languages do not provide a well-founded conceptualization of the domain and are not consensual. This may lead to ambiguous or weak understanding of GORE concepts. In this paper, we introduce the Goal-Oriented Requirements Ontology (GORO), a domain ontology founded on the Unified Foundational Ontology (UFO) that intends to represent the nature and relations of concepts surrounding the GORE domain. We use GORO to explore and clarify the semantics used, sometimes implicitly, by well-known GORE languages.