Ontology.lol

What is the difference between a taxonomy and an ontology?

A taxonomy sorts things into a hierarchy of types and answers "what kind of thing is this?"; an ontology adds named relationships between things and answers "how does this thing relate to those things?". The W3C standard for taxonomies is SKOS (Recommendation, 18 August 2009) and the W3C language for ontologies is OWL 2 (Recommendation, 11 December 2012). Schema.org, the vocabulary behind most website structured data, is an ontology in this sense, with 826 types and 1,540 properties in version 30.1 (16 September 2026).

What is the main difference between a taxonomy and an ontology?

The main difference is relationships. A taxonomy is a tree of types: Services > Roofing > Repair > Leak repair. A taxonomy answers "what kind of thing is this?". An ontology keeps the types and adds named relationships that cross the tree: the leak-repair service is offered by the Miami branch, the Miami branch is part of the company, the company is located in Florida. An ontology answers "how does this thing relate to those things?".

Taxonomy, ontology and schema.org side by side

Taxonomy Ontology Schema.org
Question it answers What kind of thing is this? How does this thing relate to those things? Which shared words describe this thing and its links?
W3C standard SKOS, Recommendation, 18 August 2009 OWL 2, Recommendation (Second Edition), 11 December 2012 None: a community vocabulary, version 30.1, 16 September 2026
Links between items skos:broader, skos:narrower, skos:related Any named property between classes, individuals and data values 1,540 properties, each with expected types (domainIncludes and rangeIncludes)
Hierarchy Broader and narrower concepts; skos:broader is not transitive Classes and subclasses 826 types in a multiple-inheritance hierarchy: LocalBusiness is a subtype of both Organization and Place
Formal meaning "Not a formal knowledge representation language" (SKOS Reference) "Formally defined meaning" (OWL 2 Overview) Lenient: consumers "are not obliged to treat unexpected structures as errors" (data model)
A business example Services > Roofing > Repair > Leak repair The Miami branch offers leak repair; the branch is part of the company RoofingContractor, areaServed, parentOrganization

Sources: the SKOS Reference, the OWL 2 Overview, schema.org's schemas page, data model and vocabulary file (version 30.1), all checked 13 November 2026.

What question does a taxonomy answer?

A taxonomy answers "what kind of thing is this?". Each item in a taxonomy gets a place in a hierarchy, from broad to narrow, so a person or a machine can sort and browse. Most businesses already have a taxonomy: the menu, the service list, the category pages. What a taxonomy can't say is who offers the service, where, or for whom. That is the ontology's job.

Which W3C standard is used to publish taxonomies?

The W3C standard for taxonomies is SKOS, the Simple Knowledge Organization System. The SKOS Reference (W3C Recommendation, 18 August 2009) describes SKOS as "a common data model for sharing and linking knowledge organization systems via the Web", covering thesauri, classification schemes, subject heading systems and taxonomies. SKOS links concepts with skos:broader and skos:narrower, and the SKOS Reference states that SKOS "is not a formal knowledge representation language".

Is skos:broader transitive?

No. In the SKOS Reference, skos:broader and skos:narrower are not declared transitive and are used by convention only for direct parent and child links. SKOS provides separate skos:broaderTransitive and skos:narrowerTransitive properties for inferring indirect links (SKOS Reference, 18 August 2009). In plain terms: "Leak repair" is narrower than "Repair", and a SKOS taxonomy does not automatically say it is narrower than "Services".

Which W3C language is used to write ontologies?

The W3C language for ontologies is OWL 2, the Web Ontology Language. The OWL 2 Overview (W3C Recommendation, Second Edition, 11 December 2012) calls OWL 2 "an ontology language for the Semantic Web with formally defined meaning".

What building blocks does an OWL 2 ontology describe?

An OWL 2 ontology describes four building blocks: classes, properties, individuals and data values (OWL 2 Overview, 11 December 2012). Classes are the types (LocalBusiness), properties are the relationships and attributes (offers, address), individuals are the specific things (your Miami branch), and data values are the literal facts (a phone number, a date).

What is the classic definition of an ontology?

The classic definition is Tom Gruber's: "An ontology is an explicit specification of a conceptualization." The wording appears in his Stanford Knowledge Systems Laboratory report of 1992, published in the journal Knowledge Acquisition 5(2), pages 199-220, in 1993 as A Translation Approach to Portable Ontology Specifications. Gruber describes a conceptualization as "an abstract, simplified view of the world that we wish to represent for some purpose", and an ontology as a description of the concepts and relationships that can exist for an agent or a community of agents.

Is schema.org a taxonomy or an ontology?

Schema.org is an ontology: it has a hierarchy of types and the properties that connect them. The schema.org data model describes "a set of types, arranged in a multiple inheritance hierarchy where each type may be a sub-class of multiple types", and each property can have one or more types as its domain and its range.

Can a schema.org type have more than one parent?

Yes. Schema.org uses multiple inheritance, so a type can be a subclass of several types (schema.org data model, checked 13 November 2026). That alone breaks the strict one-parent tree of a classic taxonomy.

How many types and properties does schema.org have?

Schema.org has 826 types, 1,540 properties, 19 datatypes, 96 enumerations and 544 enumeration members, according to its schemas page (version 30.1, dated 16 September 2026, checked 13 November 2026).

Does a business need a taxonomy, an ontology, or both?

A business needs both: the taxonomy to sort what it offers, the ontology to say how its offers, places, people and brands connect. The two fail differently. A bad taxonomy misfiles one page. A missing or wrong ontology leaves every system that reads your structured data to guess who offers what, where, and the guesses don't have to agree. Start from the taxonomy you already have, then add the relationships with schema.org properties.

Sources

Next step

See how relationships are written as data: triples, explained.