Definition 1 (RDF Graph). An RDF graph is a set of RDF triples, where the set of nodes is the set of subjects and objects of RDF triples in the graph.
The RDF graphs that do not have blank nodes are called ground RDF graphs.
Definition 2 (Ground RDF Graph). A ground RDF graph over the set of IRIs ?, set of blank nodes ?, and set of RDF literals ? is a set of ground triples over ?, ?, ?, which are elements in (? × ? × (? ∪ ?)).
The semantics of RDF graphs is fixed via interpretations, such as simple RDF interpretations and RDF interpretations.