consuetudo

Consuetudo, consuetudinis, f., third declension, from consuesco, "grow accustomed" - the noun of a habit that has settled in and worn a groove. Latin keeps it distinct from its neighbours: mos is the moral custom, the done thing, with a whiff of obligation; usus is practice or use; consuetudo is what one is simply used to, whether an individual or a whole people.

Legal Latin gives it teeth. Consuetudo is customary law, unwritten but binding, standing against lex, the written statute, and medieval charters are full of the consuetudines of a place. One trap for the unwary: consuetudo also means an intimacy, a close association between people, and quite often a sexual liaison, so a text discussing somebody's consuetudo with somebody else may not be about their filing habits. English custom and costume both descend from it through French.