Referer Header
Also known as: referrer, referer
The header that tells a site which page you were on before clicking through.
Every time you click a link, your browser tells the destination the URL you came from via the Referer header (yes, misspelled in the original spec). That leaks navigation history to every site you visit, and to every third-party resource those sites load.
Modern browsers strip or trim the Referer by default (Referrer-Policy: strict-origin-when-cross-origin) and sites can tighten it further. It's a small but meaningful privacy improvement over the historical default.