A Web site that is a replica of an already existing site, used to reduce network traffic (hits on a server) or improve the availability of the original site. Mirror sites are useful when the original site generates too much traffic for a single server to support.
Mirror sites also increase the speed with which files or Web sites can be accessed: users can download files more quickly from a server that is geographically closer to them. For example, if a busy New York-based Web site sets up a mirror site in England, users in Europe can access the mirror site faster than the original site in New York.
Sites such as Netscape that offer copies or updates of popular software often set up mirror sites to handle the large demand that a single site may not be able to handle.