Rule feature

Definition
Rule feature is a type of diacritic feature which triggers (or blocks) the application of a phonological rule. This feature is usually assumed to account for irregular word formation.

Example
Alternations such as foot:feet, goose:geese and tooth:teeth can be accounted for by assuming that the words foot, goose and tooth, have a rule feature [+U] which triggers the phonological umlaut rule.

Links

 * Utrecht Lexicon of Linguistics