HTTP Referer

The HTTP Referer is a header in the browser's request that carries the address of the page the user clicked a link on; the raw mechanism that traffic-source analysis is built on.

The first point is that this header is spelled “Referer” in the HTTP standard by a historical typo (missing the second r), while the general concept is “referrer”; that technical distinction separates it from the acquisition concept. The HTTP Referer is the raw signal: the browser sends it automatically and it exists before any mapping to a channel or source. But it is unreliable and has grown more restricted over time: a Referrer-Policy can shorten or drop it, entries from inside messaging apps often carry none, and an HTTPS-to-HTTP hop wipes it. The result is that a missing referer does not necessarily mean “typed directly”; much of it is lost referral traffic that, with its signal erased, settles into Direct. This is why the HTTP Referer should be read alongside UTM parameters, not on its own.