In Information Science, an ontology is the product of an attempt to formulate an exhaustive and rigorous conceptual schema about a domain. This domain does not have to be the complete knowledge of that topic, but purely a domain of interest decided upon by the creator of the ontology.

An ontology is typically a hierarchical data structure containing all the relevant entities and their relationships and rules within that domain (e.g., a domain ontology). However, computational ontology does not have to be hierarchical at all. The computer science usage of the term ontology is derived from the much older usage of the term ontology in philosophy.

An ontology which is not tied to a particular problem domain but attempts to describe general entities is known as a foundation ontology or upper ontology. Typically, more specialized domain specific schemata must be created to make the data useful for real world decisions.

A domain ontology is an ontology tied to a specific domain. A foundation ontology is a form of ontology that tries to be less specific, and in that way more generally applicable. It contains a core glossary in whose terms everything else in a broad domain can and must be described. An example is the 2000 words of English required by Longman's dictionary, used to define the 4000 most common English idioms. A foundation ontology in computer science would serve as core ontology for both computer programs and users, influencing their view of data and events.

A computer science example is that, by default, all computer programs have a foundation ontology consisting of a processor instruction set, standard library in a programming language, files in accessible file systems, or some other list of 'what exists'. Because these may be poor representations for certain problem domains, more specialized schema must be created to make the data useful in making real world decisions. Thus the need for standards which take 'core' ontologies (e.g. the Dublin Core in SGML) and solidify them into 'foundations'.

Tom R. Gruber and R. Studer have described an ontology in this sense as "an explicit and formal specification of a conceptualization" .