The Collaborative International Dictionary of English v.0.48:

Redundance \Re*dun"dance\ (r?*d?n"dans), Redundancy
\Re*dun"dan*cy\ (-dan*s?), n. [L. redundantia: cf. F.
   redondance.]
   [1913 Webster]
   1. The quality or state of being redundant; superfluity;
      superabundance; excess.
      [1913 Webster]

   2. That which is redundant or in excess; anything superfluous
      or superabundant.
      [1913 Webster]

            Labor . . . throws off redundacies.   --Addison.
      [1913 Webster]

   3. (Law) Surplusage inserted in a pleading which may be
      rejected by the court without impairing the validity of
      what remains.
      [1913 Webster]

WordNet (r) 3.0 (2006):

redundancy
    n 1: repetition of messages to reduce the probability of errors
         in transmission
    2: the attribute of being superfluous and unneeded; "the use of
       industrial robots created redundancy among workers" [syn:
       redundancy, redundance]
    3: (electronics) a system design that duplicates components to
       provide alternatives in case one component fails
    4: repetition of an act needlessly

The Free On-line Dictionary of Computing (19 January 2023):

redundancy

   1.  The provision of multiple
   interchangeable components to perform a single function in
   order to provide resilience (to cope with failures and
   errors).  Redundancy normally applies primarily to hardware.
   For example, a cluster may contain two or three computers
   doing the same job.  They could all be active all the time
   thus giving extra performance through parallel processing
   and load balancing; one could be active and the others
   simply monitoring its activity so as to be ready to take over
   if it failed ("warm standby"); the "spares" could be kept
   turned off and only switched on when needed ("cold standby").
   Another common form of hardware redundancy is disk
   mirroring.

   2.  data redundancy.

   (1995-05-09)