aka semantic network, semantic web
There are a lot of fuzz around the subject and there is no one widely recognised definition. But here is definition which is good in a given context:
KG definition
Knowledge graph - a Graph of data intended to accumulate and convey knowledge of the real world, whose nodes represent entities of interest and whose edges represent potentially different relations between these entities.
Knowledge. While many definitions for knowledge have been proposed, we refer to what Nonaka and Takeuchi call “explicit knowledge,” i.e., something that is known and can be written down
From https://dl.acm.org/doi/fullHtml/10.1145/3447772 (https://arxiv.org/pdf/2003.02320.pdf).
Other definitions of KG:
- Towards a Definition of Knowledge Graphs
- What is a Knowledge Graph?
- Knowledge Graphs
- An Introduction to Knowledge Graphs
Ontology definition
In computing, an ontology is then a concrete, formal representation—a convention—on what terms mean within the scope in which they are used (e.g., a given domain). Like all conventions, the usefulness of an ontology depends on how broadly and consistently it is adopted and how detailed it is. Knowledge graphs that use a shared ontology will be more interoperable.
From https://dl.acm.org/doi/fullHtml/10.1145/3447772 (https://arxiv.org/pdf/2003.02320.pdf).
Again a lot of fuzz around the subject. Based on some descriptions it is something magical. I rather prefer to think about it as knowledge graph about other knowledge graphs. And in this sense ontologies can form a hierarchy. And there are top-level ontologies - the most abstract ontologies.
Top level ontologies
aka Upper, Foundational, Generic
- UFO Unified Foundational Ontology
- gUFO: A Lightweight Implementation of the Unified Foundational Ontology
- BFO Basic Formal Ontology
- DOLCE Descriptive Ontology for Linguistic and Cognitive Engineering
- SUMO Suggested Upper Merged Ontology
- YOMATO Yet Another More Advanced Top-level Ontology
- GFO General Formal Ontology
- PROTON PROTo ONtology
- Upper Cyc Ontology
- BORO Ontology Business Object Reference Ontology
- CIDOC Conceptual Reference Model
- EuroWordNet Top Ontology, Vossen 1998
- IEEE Standard Upper Ontology
- Mikrokosmos Ontology
- OntoUML
Scientific papers
- A Comparison of Upper Ontologies
- Toward the Use of an Upper Ontology for U.S. Government and U.S. Military Domains: An Evaluation
- Qualities, Quantities, and the Endurant-Perdurant Distinction in Top-Level Ontologies
- Foundational Ontologies meet Ontology Matching: A Survey
- Applicability of a Foundational Ontology to Semantically Enrich the Core and Domain Ontologies
- How Well are Domain and Upper Ontologies Connected?
- Validating Top-level and Domain Ontology Alignments using WordNet
- Foundational Ontologies
- Thing Foundational Ontology: ThingFO v1.3’s Terms, Properties, Relationships and Axioms
- Matching Domain and Top-level Ontologies via OntoWordNet
- Missing lateral relationships in top-level concepts of an ontology
- Towards a Standard Upper Ontology
- A Comparison of Upper Ontologies
- A survey of Top-Level Ontologies
- Towards a First-Order Modal Formalisation of the Unified Foundational Ontology