In computer science and artificial intelligence, ontology languages are formal languages used to construct ontologies. They allow the encoding of knowledge about specific domains and often include reasoning rules that support the processing of that knowledge. Ontology languages are usually declarative languages, are almost always generalizations of frame languages, and are commonly based on either first-order logic or on description logic.

Classification of ontology languages

Classification by syntax

Traditional syntax ontology languages

Markup ontology languages

These languages use a markup scheme to encode knowledge, most commonly with XML.

Controlled natural languages

Open vocabulary natural languages

Classification by structure (logic type)

Frame-based

Three languages are completely or partially frame-based languages.

Description logic-based

Description logic provides an extension of frame languages, without going so far as to take the leap to first-order logic and support for arbitrary predicates. Gellish is an example of a combined ontology language and ontology that is description logic based. It distinguishes between the semantic differences among others of: It also contains constructs to express queries and communicative intent.

First-order logic-based

Several ontology languages support expressions in first-order logic and allow general predicates.