redundant(ri-dun´d&nt) (adj.) Used to describe a component of a computer or network system that is used to guard the primary system from failure by acting as a back up system. Redundant components can include both hardware elements of a system -- such as disk drives, peripherals, servers, switches, routers -- and software elements -- such as operating systems, applications and databases.
Redundancy is the quality of systems or elements of a system that are backed up with secondary resources. For example, "The network has redundancy."