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
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.