An orphan page is live and reachable by direct URL, and nothing on the site links to it. Navigation does not include it, no article references it, and no listing shows it.
Crawlers find such pages only through the sitemap or an external link, so discovery is slow and recrawling is infrequent. More importantly, internal links are how authority moves around a site, and a page with no inbound internal links receives none of it. Search engines also read internal linking as a signal of what the site considers important, and an orphan says nothing.
They accumulate quietly. Campaign landing pages outlive the campaign, a category gets removed from navigation while its pages remain, a CMS migration drops links, or a page is published before the linking edit that was meant to follow.
Finding them takes two lists. Crawl the site from the homepage to get everything reachable by link, then compare against the full URL inventory from the sitemap, the CMS, and server logs. Anything in the second list and not the first is orphaned, and each one gets either a real internal link or a decision to remove it.