In my opinion, the names "tag wiki" and "tag wiki excerpt" are misnomers. Why?
- In most wikis (that allow the community to edit them), very little permission is needed to edit each of its pages. For tag wikis, though, a whopping 20,000 reputation is required to edit most tag wikis without approval, whereas for Community Wiki posts, the requirement is merely 100 reputation.
- Strictly speaking, an "excerpt" is a small text quoted from a larger text. However, many "tag wiki excerpts" are not "excerpts" per se, but rather more like summaries.
For these reasons, I recommend calling "tag wikis" and "tag wiki excerpts" by a different name.
I prefer "tag details" and "tag summary" myself, but there may be other, better names.
(I am not suggesting that the way tag wikis work should be changed beyond their names.)