Orphan page
An orphan page is one linked by no internal link: it may exist in the sitemap, but no navigation path leads to it. Crawlers discover it poorly and assign it little value.
In practice
Orphans are created by redesigns, forgotten campaigns, or product pages removed from the catalog but not from the server. Detection cross-references three lists: sitemap URLs, URLs seen in server logs, and URLs reachable by crawl. What deserves to live gets reconnected to the link structure; the rest gets redirected or removed.