Foundations · 12 min read
Taxonomy is not ontology.
Practitioners often use "taxonomy" and "ontology" as if they were synonyms. They are related, but each makes a claim the other does not — and conflating them produces classification systems that collapse on contact with edge cases.
A taxonomy is a hierarchy. Things have parents and children and siblings. The set of relationships expressible inside a taxonomy is intentionally narrow: is-a, occasionally part-of. Dewey Decimal is a taxonomy. So is the Linnaean system.
An ontology is a graph. It admits arbitrary typed relationships: treats, causes, is-derived-from, was-issued-by. Schema.org is an ontology. Wikidata is an ontology so vast that it indexes most of human knowledge in subject-predicate-object triples.
Why the distinction matters
A taxonomy tells you where to file something. An ontology tells you what it can do.
Quick reference
Most working classification systems are taxonomies with an ontology glued on. NAICS is hierarchical, but its mappings to ISIC and SIC form a graph. The hierarchy gets you to the right neighborhood; the graph tells you what the neighborhood means.