The best description I can find is in the the type description:
“A word, name, acronym, phrase, etc. with a formal definition. Often used in the context of category or subject… ”

So I would see it used in the context of an article for instance, as the value of a subject:
“about”: {
“@type”: “DefinedTerm:,
“@id”: “http://example.org/Sub/1”,
“name”: “The Subject”,
“sameAs”: “http://wikidata.org/….”,
}

Yes you are correct that you are creating and/or referencing your/another vocabulary. This really comes into play when you/others start reusing these terms across pages/sites.

A good example of this would be to define the categories/tags on a blog as DefinedTerms (on the category page(s)), referencing them to global equivalents, such as wikidata etc, and from each post.

An excellent question Henk!

Since the early days of Schema.org there have been several “why don’t you support / integrate this or that ontology / vocabulary” debates on the lists.

Two well, and often, one debated have been FRBR and SKOS.

The style depth and associated baggage that such full adoption would bring to the Schema.org vocabulary, and the potential unintended consequences in complexity etc, for the target adopters, I believe were the main reasons for these debates to die out without concrete action.

There was another attempt in 2013 to introduce a miniSKOS by introducing a Topic type which would have been an equivalent Class to skos:Concept, but that also went nowhere – there is an understandable reticence to tie Schema types to classes in other vocabularies.

The introduction of DefinedTerm and the associated CategoryCode was an initiative from myself to satisfy the needs of those that have lists of terms/concepts/etc. that they want to share on the web (without having to share all the management stuff that often comes with SKOS – or get their head around SKOS in the first place).

The presumption being that if you need SKOS to organise all your knowledge, you will already have it, and adding a very thin layer of Schema.org on top will make it widely sharable.

This is the same pattern as for all domains – manage your data with the ontology/vocabulary most appropriate for your domain, then add some Schema.org to make it generically consumable across all domains via a web presence.

That’s the short answer, I’m more than pleased we got DefinedTerm into the vocabulary to at least satisfy a need. I am interested to see the many and varied ways it will be adopted.

~Richard

Thanks for spotting the trailing comma – now fixed.

The 3.4 release of Schema.org is only currently a few days old. I cannot speak for Google, or any of the other major consumers of Schema.org. However, it is usually a few weeks before they start to recognise updates in the vocabulary, either in their interpretation of data or in their public testing tools.