1. <hardware, storage> 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.
Several operating systems support software disk mirroring or disk-duplexing, e.g. Novell NetWare.
See also Redundant Array of Independent Disks.
Interestingly, when this technique is used with magnetic tape storage systems, it is usually called "twinning".
A less expensive alternative, which only limits the amount of data loss, is to make regular backups from a single disk to magnetic tape.
2. mirror site.
Try this search on Wikipedia, OneLook, Google
Nearby terms: Miracula « Miranda « MIRFAC « mirror » mirroring » mirror site » MIS