A custom or usage that has acquired the force of law

Wiktionary Word definitions in Wiktionary
n. custom, familiarity.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
late 14c., from Middle French consuetude , from Latin consuetudo , from consuetus , past participle of consuescere "to accustom" (see custom ).

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Consuetude \Con"sue*tude\ (?; 144), n. [L. consuetudo. See Custom .] Custom, habit; usage. [R.] To observe this consuetude or law. --Barnes .

WordNet Word definitions in WordNet
n. a custom or usage that has acquired the force of law