Internet of Things (IoT) Ontology is a reference ontology for data integration and semantic coordination of smart entities. The aim of the ontology is to provide a clear understanding of the new research domain of IoT in respect to the need for 'true' (i.e. semantic) interoperability of smart entities and other kind of entities (control, physical) that may be plugged in it anytime, by anyone and from anyplace. The objective is not to focus in sensor and observation data descriptions as in SSN ontology, but instead to emphasize the notion of interconnected, clustered andaligned smart entities towards supporting their semantic registration, coordination and retrieval in a Web of Things.