Referrer

The referrer is the address of the page a user clicked a link on to reach your site; the browser sends it in the Referer header along with the request.

The referrer is the rawest acquisition signal and exists before any mapping; channel and source are built from it afterward. But it is unreliable: depending on the Referrer-Policy the browser may shorten or drop it, entries from an app (a click inside Instagram or Telegram) often carry none, and an HTTPS-to-HTTP hop wipes it too. The consequence is that a missing referrer does not necessarily mean “typed the address directly”; much of it is lost referral traffic that, with its signal erased, settles into Direct. This is why the referrer should be read alongside UTM, not on its own.