Originally an acronym for Redundant Array of Independent Disks, the term RAID is now used more broadly to refer to a wide variety of digital data storage structures that subdivide and reproduce data across a series of separate physical drives. RAID is a form of storage virtualization that enables an operating system to access the entire array of disks as though it were a single hard drive.
While not a replacement for or alternative to backing up data, a RAID can prevent catastrophic data loss caused by damage or loss of a single component within the RAID itself, making it preferable to storing data on a single drive. An entire RAID can be lost to physical damage and is vulnerable to malicious data deletion and hacking, so even when using a RAID, backing up data off-site is still necessary to provide true data redundancy.
When it was first coined, RAID stood for Redundant Array of Inexpensive Disks, but RAID manufacturers and distributors later encouraged the industry to change the “I” in RAID from inexpensive to independent, in order to discourage purchasers from having an expectation of a low price for RAID technologies.