Ontology

In information science, an ontology is a formal, explicit description of the concepts in a domain and the relations between them. Tom Gruber's often quoted definition calls it "an explicit specification of a conceptualization". Unlike a Taxonomy, an ontology isn't limited to broader and narrower terms: it defines classes, properties, and rules that software can reason with.

Building blocks

• Classes — kinds of things: Product, Component, Safety instruction
• Instances — individual things: Pump P200
• Properties — relations and attributes: hasComponent, appliesTo, replaces
• Axioms — rules, such as "every warning message applies to at least one hazard"

The W3C standards RDF, RDFS and OWL provide the formal languages; SKOS covers the lighter, thesaurus-like end of the spectrum (see Controlled vocabulary).

Ontology vs. taxonomy vs. knowledge graph

• A Taxonomy organizes terms in a hierarchy.
• An ontology defines what kinds of things and relations exist.
• A Knowledge graph is the data: many instances connected according to an ontology.

In technical documentation

Ontologies make documentation metadata meaningful across systems. iiRDS, for example, is an RDF-based standard whose ontology defines information types, product metadata, lifecycle phases and more, so that documentation from different suppliers can be combined and filtered in one portal. See the information architecture and ontology glossary for the terms.