A collection of collections and a specialization of empty set or collection (q.v.). A collection is an instance of empty collection if and only if it has no instances.

Some collections are by their very nature necessarily empty; e.g. non-thing (q.v.). Conversely, some collections are empty in some contexts and non-empty in others; e.g. unicorn might be empty in the context of the actual world but non-empty in certain mythological contexts. Similarly, a time dependent collection might be empty at certain times and non-empty at certain other times; e.g. the collection of female US presidents is currently empty but will hopefully become non-empty some time in the near future. This last example demonstrates that empty collection is itself a time-dependent collection.

Note that each instance of empty collection is Disjoint With (q.v.) every collection -- including itself.

Cf. non empty collection.