What is an Ontology

I need to provide some quick definitions, starting with ontology. It is a rich irony that the word “ontology”, which has to do with making clear and explicit statements about entities in a particular domain, has so many conflicting definitions. I’ll offer two general ones.

The main thread of ontology in the philosophical sense is the study of entities and their relations. The question ontology asks is: What kinds of things exist or can exist in the world, and what manner of relations can those things have to each other? Ontology is less concerned with what is than with what is possible.

The knowledge management and AI communities have a related definition — they’ve taken the word “ontology” and applied it more directly to their problem. The sense of ontology there is something like “an explicit specification of a conceptualization.”

The common thread between the two definitions is essence, “Is-ness.” In a particular domain, what kinds of things can we say exist in that domain, and how can we say those things relate to each other?

Clay Shirky: Ontology is Overrated: Categories, Links, and Tags

Tagging vs. semantic Web


Tagged content does not equal structured data.


  • are labels with words or a combination of words that are attributed/attached to pieces of content – most often retrospectively by users;
  • describe content implicitly and hence are ambiguous (like any term in human language)
  • ;

  • tagging is a light version of the semantic Web.
    The semantic web

  • is like a logical layer that sits on top of the Web (of documents);
  • the semantic Web is a Web of structured data and a data-field is the smallest semantic unit in the Web;
  • using (standard) identifiers and expressions of relationship between data fields, description of content is made explicit and non-ambiguous;
  • the semantic Web is based on interoperable metadata standards such as identifiers and expressions of relationships.

The way from the Web of Documents to the Semantic Web requires a shift in thinking: from “thinking repository scale to thinking Web scale” (Silver Oliver)

Side note: Interestingly, according to semiotic thinking, human language terms (as utilised in tags) become meaningful in differentiation to competing semantic units, i.e. by implicit logical exclusion – ‘is not this, is not that’ – whereas in the semantic Web meaning is defered by explicit attribution of identifier and a limited number of expressions of relationships.

Semantic Web: Trends, Projects, Links

Some acronyms that will rock the Web (or maybe not):