Sometimes, it might be useful to look up a subject in the same way you would do so in a book—in the index. Wikipedia has an index. To use it—it is available at the editor's index to Wikipedia.
Although it is called an "editor's index", it is really intended to be an index for anyone involved with Wikipedia in other than the role of a reader. Most of the topics are directly relevant to editing of articles, but some topics are less directly relevant to editing.