1. A group of hard disks under the control of array management software that work together to improve performance and decrease the odds of losing data due to mechanical or electronic failure by using such techniques as data striping. RAID implementations, because of their complexity and steep cost, are most often used on network servers. Several RAID levels exist, each with advantages and disadvantages. See RAID level 0 through RAID level 53. From QUECID

2. Redundant Array of Inexpensive Disks - a method whereby information is spread across several disks, using techniques such as disk striping (RAID Level 0) and disk mirroring (RAID level 1) to achieve redundancy, lower latency and/or higher bandwidth for reading and/or writing, and recoverability from hard-disk crashes. From Linux Guide @FirstLinux