The Free On-line Dictionary of Computing (30 December 2018):

data redundancy

    Any technique that stores or
   transmits extra, derived data that can be used to detect or
   repair errors, either in hardware or software.  Examples are
   parity bits and the cyclic redundancy check.

   If the cost of errors is high enough, e.g. in a
   safety-critical system, redundancy may be used in both
   hardware AND software with three separate computers programmed
   by three separate teams ("triple redundancy") and some system
   to check that they all produce the same answer, or some kind
   of majority voting system.

   The term is not typically used for other, less beneficial,
   duplication of data.

   2.  The proportion of a message's gross
   information content that can be eliminated without losing
   essential information.

   Technically, redundancy is one minus the ratio of the actual
   uncertainty to the maximum uncertainty.  This is the fraction
   of the structure of the message which is determined not by the
   choice of the sender, but rather by the accepted statistical
   rules governing the choice of the symbols in question.

   [Shannon and Weaver, 1948, p. l3]

   (2010-02-04)