Disambiguation: SubWiki is also a term used for a wiki based upon subversion. See SubversionWiki?.
A subwiki as a part of a wiki – a NameSpace; a main page and its subpages; a branch of an hierarchical wiki. The subwiki is as independent from the superwiki as possible. It should be treated like a wiki of its own in as many ways as possible.
See WikiSubcommunity for the social implications of subwikis.
See FractalWiki for a discussion of the entire idea of having wikis within wikis.
See WikiNameSpace for a more in-depth discussion of namespaces. The rest of this page will focus on subwikis.
PmWiki has the capability of classifying pages into groups of related pages. WikiWords form links between pages of the same group, ie. the group acts as a namespace. You can link to pages in other namespaces using a
GroupName/PageName. A page group can be password protected, thus allowing private subwikis for individuals (WikiAsPim) or small groups. 
Comparing the PmWiki page groups to SubPages: Sub-pages require you to explicitly create a main page for the group. RecentChanges is usually not available for pages in that group only. Ordinary page names link into the global namespace per default. Links to pages in the same group require a slash as a prefix:
/PageName. To link to another group, the syntax is practically identical: