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.

Another common use of the term 'mirror' refers writing duplicate data to more than one device (usually two hard disks), in order to protect against loss of data in the event of device failure. This technique may be implemented in either hardware (sharing a disk controller and cables) or in software. It is a common feature of RAID systems.