Orphan Page
A page with no crawlable internal links pointing to it from the rest of the website.
Orphan pages are hard for search engines and visitors to find, so valuable content or landing pages can go unindexed or unvisited, while forgotten orphan pages may expose outdated offers.
An SEO lead exports 2,400 URLs from the XML sitemap and analytics, compares them with 2,150 URLs found by a site crawl, and reviews the 250 missing ones, linking useful pages from category hubs and redirecting obsolete ones.
A page linked only through JavaScript events, forms or a sitemap may still count as orphaned for crawlers. Some orphans are intentional, such as campaign landing pages, and adding links without context does little for users.
Search engines discover most pages by following links, generally a elements with an href attribute, so a page without internal links depends on sitemaps or external links to be found. Orphans are identified by comparing URLs known from sitemaps, analytics, server logs or Search Console against URLs reachable in a crawl from the home page. Fixes are adding contextual links, redirecting or removing the page.
Systems Architecture
Google Search Central — SEO Link Best Practices for Google — https://developers.google.com/search/docs/crawling-indexing/links-crawlable; Ahrefs — How to Find and Fix Orphan Pages (The Right Way) — https://ahrefs.com/blog/orphan-pages/