Redundancy is a relational trait of a set of elements which each perform the same non-accumulative function within a specific context.
Redundancy is a type of replaceability. Redundancy implies that replaceable elements are intentionally held in reserve, purely as a safeguard against disruptions.
Redundancy can be inefficient, but it can also increase the security of systemic functions. Depending on context, some redundancy is usually required for system security.