Redundancy rule

Redundancy rule is rule which fills in predictable or redundant information. Redundancy rules have two important properties: (a) they do not create structure, and (b) they do not alter structure.

Example
the fact that sonorants in English are always voiced, as opposed to obstruents, can be captured by leaving the feature [voice] unspecified, and fill in [+voice] by a redundancy rule. The idea behind redundancy rules and underspecification is that redundant information can be left unspecified in the grammar (usually the lexicon), and that a grammar which contains less (idiosyncratic) information is more highly valued than a grammar which contains more (every thing else being the same).

Links
Utrecht Lexicon of Linguistics