The InterMap is a system for defining links between WikiWikiWeb sites that was first developed by UseMod:UseModWiki and Meatball:MeatballWiki. InterMap links have the form
MapPrefix:Page, where the host prefix is converted to a partial URL based on entries in the site's intermap.txt and localmap.txt files.
The default intermap.txt distributed with PmWiki includes the following InterMap entries:
PmWiki http://www.pmwiki.org/pmwiki2/pmwiki.php/ Cookbook http://www.pmwiki.org/pmwiki2/pmwiki.php/Cookbook/ Wiki http://www.c2.com/cgi/wiki? UseMod http://www.usemod.com/cgi-bin/wiki.pl? Meatball http://www.usemod.com/cgi-bin/mb.pl? Wikipedia http://www.wikipedia.com/wiki/
Thus, PmWiki:Variables becomes a link to the PmWiki.Variables page on the official PmWiki web site, Wiki:FrontPage is a link to the front page of the first WikiWikiWeb, and Wikipedia:Stonehenge takes you to the Wikipedia article about the famous megaliths in England.
InterMap links can be used to link to non-wiki pages: ISBN:020171499X is a link to Amazon.com's entry for The Wiki Way by Bo Leuf and Ward Cunningham (an excellent book about WikiWikiWebs in general).
The actual set of InterMap links at any site is defined by the site administrator; for more details, see PmWiki.CustomInterMaps.