Definition
The referrer is the previous page URL a browser may send when it requests your page. The header is spelled Referer.
A referrer is the address of the previous page, if the browser chose to send it. You learn the site that linked to you. You do not learn the search query, which Google stopped sending, or which button on that page was clicked. The HTTP header is spelled Referer. Reports spell it correctly and mean the same field.
When the header is missing, the visit is direct traffic: bookmarks, typed addresses, and apps that hide the source. Missing is normal. Missing on every visit, including a link you just clicked from a site you control, means something stripped the header.
MapMyVisitors stores the referrer on the visit. Reading the list is how to read a referrer report. The question form is what is a referrer.
