Referer

The Referer field must not be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard.

If a referring URL was specified, for each valid referer, make sure that a valid referring URL was passed to FormMail

The referer portion of a logfile line contains the full URL of the page or object visited before the current one.

The referer information can also be accessed as document.referrer within JavaScript.