Tags that are placed in some form of hierarchy, not necessarily in one hierarchy alone.
When organizing information, it is common to use either a hierarchy or tags. An element in a hierarchy is ordered within a predefined category and relates to other elements above, below or at the same level. The only direct link between elements in a hierarchy is between the immediate superior and subordinate.
Tags, on the other hand, are non-hierarchical keywords assigned to an element. There are no links between tags, and elements can have tags on different hierarchical level, which in a strict hierarchy would not be possible.
Tag-hierarchy refers to tags that are placed in some form of hierarchy, not necessarily in one hierarchy alone.
Questions having the tag-hierarchy tag regards tags in hierarchies.