orphans directive is supplied by the orphans plugin.
This directive generates a list of possibly orphaned pages -- pages that no other page links to. Example:
[[!orphans pages="* and !blog/*"]]
The optional parameter "pages" can be a PageSpec specifying the pages to check for orphans, default is search them all.
Note that it takes backlinks into account, but does not count inlining a page as linking to it, so will generally count many blog-type pages as orphans.