Referrer

Referrer is a term used in computing and in everyday language with several related meanings. In web and internet contexts, it most often refers to the source URL that led a user to the current page or resource. In analytics and marketing, the referrer denotes the site or page that directed traffic. In general English, a referrer can also be a person who refers someone to a service or professional.

In web communications, the HTTP header commonly associated with referrers is called Referer, a historical misspelling that persists in the protocol. When a user agent requests a resource, it may include the Referer header containing the URL of the page that initiated the request. Not all requests include it, and users or privacy tools may restrict or remove it. The header helps with analytics, logging, and sometimes decision making in downstream processing, but it also raises privacy considerations because it can reveal sensitive information from the referring page.

Referrer policy is a mechanism to control how much referrer information is sent. The policy can be delivered via a HTTP Referrer-Policy header or a meta tag and specifies values such as no-referrer, origin, same-origin, origin-when-cross-origin, strict-origin-when-cross-origin, and unsafe-url. These settings balance the utility of referrer data against privacy and security concerns, particularly for cross-origin requests.

Overall, the concept of a referrer encompasses the source that led to a web request, the privacy controls that govern its disclosure, and its role in analytics and referrals in broader contexts.