Separatism is a wiki-philosophy related to inclusionism. It is the opposite of Mergism. Rather than merging everything related to one subject into a single long article, Separatists believe that while much information may be included in a related "main article", there is nothing wrong with having a separate page for every notable subject.
Wiki is not paper, and this enables it to host a vast variety of topics and information. Sometimes, people propose merging a variety of different subjects into one article because they feel that the subjects don't warrant their own article. This is something Deletionists tend to do. However, Separatists feel that there is no need to do so, because it is only natural and logical that every (notable) subject has its own article. And if an article should be deleted, there is no reason to merge it with related articles. The information remains the same, whether it is in a separate article or merged with a more general article. Merging is not a substitute for deletion. Also, subjects that fail to meet the notability criteria don't suddenly become notable when they don't have a separate article but are placed in a list.
For example, w:Supporting Harry Potter characters lists about 20 different characters. Why not give them all their own, short article, and link to them in a List of characters in the Harry Potter books or a Category:Harry Potter characters? If someone searches for "Gabrielle Delacour", they are likely to be looking for information on Gabrielle Delacour only, and not information on "Caractacus Burke" or "Fridwulfa" (and a section redirect takes them to the appropriate place in the list article). Creating a separate article for persons in a list like that doesn't hurt anybody. Also, it is a subjective matter to decide which characters are "minor" and which aren't.
Separatists also realize that merging is usually against the natural growth process of Wikipedia articles: If e.g. a novel and its film and stage play adaptations start out as separate stubs and then get merged at some point, it is usually only a matter of time before they are split again as each section in the merged article expands and gets its own references. Thus, the merged article intermediate stage has done little more than obfuscate the edit history, which would have been avoided entirely by keeping the articles separate in the first place.
Separatists maintain that while much information can also be included in a general article or list on the subject, there is no real reason to do so. They generally prefer categories over lists. Some Separatists also believe that content forking isn't harmful, as long as the forks do not take a firm point of view on a matter.
Separatism takes a negative stance on making redirects for neighbour (or closely related) notions and synonymical terms. While these can be useful in trying to avoid duplicate articles, they often get confusing or leave out important information.
See also 
- Anonymous User Protection Squad (AUPS)
- Transwiki:Constructionism and reductionism (wiki)
- Association of Inclusionist Wikipedians
- Association of Deletionist Wikipedians
- Association of Mergist Wikipedians
- Association of Wikipedians Who Dislike Making Broad Judgements About the Worthiness of a General Category of Article, and Who Are In Favor of the Deletion of Some Particularly Bad Articles, but That Doesn't Mean They are Deletionist
- Compulsive hoarding
- Proposed policy for wiki closure