OWL
OWL (Web Ontology Language) is a knowledge-representation language for authoring ontologies — formal vocabularies that define classes, properties, individuals, and the logical relationships among them. It was standardized by the W3C, first as a Recommendation in 2004 and revised as OWL 2 in 2009 (with the current edition dating to 2012). OWL grew out of earlier efforts (DAML, OIL, DAML+OIL) to bring AI knowledge representation to the Web, and it extends the more limited vocabulary of RDF Schema.
OWL is layered directly on top of RDF: every OWL ontology can be read as an RDF graph of triples, which places it squarely in the graph-data ecosystem. Its normative interchange syntax is RDF/XML (see RDF/XML), but it is commonly written in Turtle, and can be serialized as JSON-LD, N-Triples, or the RDF-friendly N3; embedded variants like RDFa also apply. OWL additionally defines non-RDF syntaxes (OWL/XML, Manchester, and functional-style) aimed at editors and reasoners.
OWL's semantics are grounded in description logic, enabling automated reasoners to infer implied facts, check consistency, and classify hierarchies. OWL 2 offers profiles (EL, QL, RL) that trade expressiveness for tractable, scalable reasoning. It is widely used in biomedical ontologies, linked data, and enterprise knowledge graphs. Tooling includes the Protege editor, reasoners such as HermiT and Pellet, and libraries like Jena/RIOT, RDFLib (Python), and dotNetRDF; conversion between serializations is possible with rdf2rdf. Limitations include the undecidability of OWL Full, expressiveness-versus-performance trade-offs, and awkward handling of n-ary relations.
Alternative Names: Web Ontology Language
Specification| Feature | OWL |
|---|---|
| Multiple Graphs per Document |  |
| Nodes |  |
| Undirected Edges | |
| Directed Edges | |
| Hyperedges | |
| Mixed-directionality Edges | |
| Parallel Edges | |
| Self-loops | |
| Edges on Edges |  |
| Nested Graphs in Nodes | |
| Nested Graphs in Edges | |
| Nested Graphs in Graphs | |
| Node Labels | |
| Edge Labels | |
| Attributes on Nodes | |
| Attributes on Edges | |
| Attributes on Graphs | |
| Typed Edges | |
Write-only Tools
Frequently Asked Questions
What is a OWL file?
A OWL file stores a graph — its nodes, edges and attributes — in the OWL format (also: Web Ontology Language). See the feature table above for what it supports.
How do I open a OWL file?
Open it in a graph tool that supports OWL, or convert it to a format your tool reads. With GraphInOut you can convert OWL to GraphML, DOT, Connected JSON and more, right in your browser.
How do I convert a OWL file to another format?
Use the Convert from OWL link above: upload or paste your OWL file (input preset to OWL), choose a target format and download the result — free, no install.