The Free On-line Dictionary of Computing (30 December 2018):

mirror site

    An archive site or website which keeps a
   copy of some or all files at another site so as to make them
   more quickly available and to reduce the load on the source
   site.

   It is generally best to use the mirror that is physically
   closest to you as this will usually give the fastest download.

   Such mirroring is usually done for specific whole directories
   or files on a specific remote server as opposed to a cache
   or proxy server which keeps copies of everything that is
   requested via it.

   For example, src.doc.ic.ac.uk is the main UK mirror for the
   GNU archive at gnu.org.

   (2006-10-16)