Taxonomy, Thesaurus, or Ontology? – Clearing the Confusion Part 1

By Marjorie Hlava, Chief Science Officer

Should your organization use a thesaurus, taxonomy, or ontology to make its information assets searchable and retrievable? Let’s talk about the differences.

Taxonomy

A taxonomy is a system of classifying and organizing data into a hierarchical structure based on shared characteristics. The NISO Z39.19 standard defines a taxonomy as “a collection of controlled vocabulary terms organized into a hierarchical structure.” The primary purpose of a taxonomy is to provide navigational structure for the user and to guide searchers to the desired concept. Each term in a taxonomy is in one or more parent/child (broader/narrower) relationships to other terms in the taxonomy.

Thesaurus

A thesaurus builds on the hierarchy of a taxonomy, focusing on listing synonyms and related terms, primarily for human-driven information retrieval. ISO 25964-1 standard defines a thesaurus as “a controlled and structured vocabulary in which concepts are represented by terms, organized so that relationships between concepts are made explicit, and preferred terms are accompanied by lead-in entries for synonyms or quasi-synonyms.” The purpose of a thesaurus is to guide both the indexer and the searcher to select the same preferred term or combination of preferred terms to represent a given concept. For this reason, a thesaurus is optimized for human navigability and terminological coverage of a domain.

Ontology

An ontology defines knowledge with explicit relationships and properties to enable computer interpretation through the meaning of terms and how they relate to each other through rules. According to Oxford Languages, an ontology is a set of concepts and categories in a subject area or domain that shows their properties and the relations between them. The ISO definitions vary by the organization defining them. ISO 21828 says an ontology is a “formal representation of phenomena of a universe of discourse with an underlying vocabulary including definitions and axioms that make the intended meaning explicit and describe phenomena and their interrelationships.” This definition, found in standards like ISO 19101-1, is “a formal, machine-readable description of concepts and their relationships within a specific domain, used for information systems and data sharing.” The purpose of an ontology is to formally define a domain’s concepts, properties, and the relationships between them, creating a shared understanding and enabling systems to reason and share information more effectively. This structured knowledge can be used for tasks like classifying information, improving search accuracy, and allowing different databases and applications to communicate with each other.

The Overlap

Although each has a formal definition and distinctions, they overlap. Ontologies generally do not have hierarchies like thesauri and taxonomies. Taxonomies do not have relationships between terms nor do they include synonyms. There are formal differences but really, with all those options for definitions, you can call your structured vocabulary the name which best suits your purpose.

The chart below (based on an answer from Microsoft AI) illustrates the differences and examples.