Selected answers from the Dumb SEO Questions G+ community.
Michael Martinez: While there is no standard definition for the phrase, I generally mean "content that is no longer being linked to from within a Website" but which may, for whatever reason, be found in search results or through links on other sites. People are most likely to create orphaned content either through PageRank traps (articles written for the sake of attracting links but which do not match the main Website) or through redesigns that overlook old content. You resolve the issue either by adding the orphaned content into your site navigation (it does not have to be sitewide navigation) or by removing the content. Alternatively, you could just leave it as a standalone microsite.