It’s always fun when misspellings make it into specs - I wonder how
many times that’s taken place. I know we have a bunch of misspelled
class names in-house…that always kills me.

Dictionary.com definition of “referer”:

A misspelling of “referrer” which somehow
made it into the HTTP standard. A given web page’s
referer (sic) is the URL of whatever web page contains the
link that the user followed to the current page. Most
browsers pass this information as part of a request.