In the Ireland Information Guide:Glossary, an orphan is defined as "A page with no links from other pages." These can still be found by searching the Ireland Information Guide, but it is preferable that they can also be reached by following links from other pages; thus it is useful to deorphan a page by adding links from suitable other pages.
There may be other ways to define the term orphan (in a Wiki context), or other related terms. This section offers various such definitions.
You can find a list of orphan articles here.