Yet another IT acronym
: "Redundant Dual Array Controller".
This is a driver that is intelligent enough to re-route requests to a device such as a SCSI disk through an alternate path should one path become unavailable. It is also possible to do load balancing between multiple paths at the same time.
Obviously this requires that there be more than one path available to the device.
There are various implementation of this model available commercailly. Most notable are Veritas' DMP and EMC powerpath.