mirror site

An alternate Web site or datacenter that contains the same information as the original. Mirror sites are set up for backup and disaster recovery as well as to balance the traffic load for numerous download requests on the Web. Such "download mirrors" are often placed in different locations throughout the Internet.

Hopefully Not a Real Mirror
It is common to use the term mirror to refer to an exact copy of data; however, a real mirror reverses characters. Stand in front of a mirror with some text on a T-shirt, and you find the letters reversed. Thus a mirror image of data is a technical misnomer, but all is fair in love and technology. See RAID and disaster recovery.

---

(networking)

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.