Orphan Pages
Orphan pages are pages on a website that no other internal page links to, which makes them hard to find both through normal navigation and for search engine crawlers. They can still exist and even be indexed, for example if they are only reachable through the XML sitemap or an external link.
In practice
Orphan pages are harder for crawlers to reach and receive no internal links to support their relevance – they therefore often perform worse, even when their content is good. A sitemap alone only helps so far: Google explicitly points out that URLs listed in it are not necessarily crawled and indexed. Typical causes are relaunches, deleted navigation items or forgotten campaign pages. You can track them down by comparing a list of all URLs from the XML sitemap or the CMS with a crawl of the pages that are actually linked. If an orphan page holds valuable content, it should be linked deliberately from thematically related pages – alternatively merge it with a redirect, or remove it if it adds nothing.