This paper provides a survey and synthesis of current efforts to define the term "knowledge graph". The authors review the literature and use their own work to develop a definition of a knowledge graph that is relevant and informative to current research in this area. Their definition of a knowledge graph is "a graph, composed of a set of assertions (edges labeled with relations) that are expressed between entities (vertices), where the meaning of the graph is encoded in its structure, the relations and entities are unambiguously identified, a limited set of relations are used to label the edges, and the graph encodes the provenance, especially justification and attribution, of the assertions."
The authors evaluate a variety of knowledge resources, graphs, and ontologies to determine if they qualify under their definition of a knowledge graph. They find that while expressing knowledge as a graph structure and unambiguous denotation of entities and relations is common, it is less common to trace the provenance of encoded knowledge and to constrain the relations used to express that knowledge. The authors have created a Knowledge Graph Catalog to support their efforts, which is available to the public for searching and contributing new knowledge graphs.