How frequently two websites (or webpages) are mentioned together by a third-party website, even if those first two items don’t link to (or reference) each other. This is a way search engines can establish subject similarity.
For instance, imagine Search Engine Journal and Search Engine Roundtable never linked to or mentioned each other. However, other websites and blogs would likely mention both SEJ and SER on lists of popular search engine news publications.
To see this in action, see: related:https://www.searchenginejournal.com/ search engine journal