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.
See also Redundant Array of Independent Disks.
Interestingly, when this technique is used with magnetic tape storage systems, it is usually called "twinning".
2. mirror site.