For now: Ontology = Tagspace
In the context of the Metajizer, the space of all tags is called the ontology.
Overblown but close
This is a bit overblown a term, since there is not much semantic information other than "[Content|DOC-9433] resources can have these Attributes, which can have these Values. It still makes sense, since
It stresses the point that we are actually gathering semantic information about our Content
The other term that is used in this context, Taxonomy, is normally associated with a strictly-hierarchic classification (even if it doesn't mean that by its definition) and could thus be misleading.