A "tag wiki" is an editable page that briefly summarizes the topic of the tag and that may provide links to existing questions that are often useful to many people.
Tag wikis are made of two parts:
- The excerpt which is displayed on question lists and the tag hover menu. It is the part shown when clicking on the "show excerpt" link in a tag wiki.
- The body which contains detailed information about a tag wiki.
When the excerpt is displayed there is a clickable item, usually labelled "Learn more" that will cause the body to be displayed.
Sometimes the term "tag-wiki" is used when referring only to the body part.
Reference: imported excerpt and wiki from the main meta Stack Exchange site.