The HTTP request header used to denote what URI linked to the resource being requested is the Referer header. This header contains the address of the previous web page from which a link to the currently requested page was followed. Note that the official specification misspells it as "Referer" instead of "Referrer"
. Additional related answers (not asked but often useful):
- The HTTP request header used to identify the client software making the request is the User-Agent header.
- The HTTP request header used to specify acceptable content types for the response is the Accept header, which is used for content negotiation.
But specifically for your query, the answer is: Referer request header.
