Posadnik was a deputy of Kniaz in some places of old Ruthenia assigned to rule a city or a land.
In Novgorod Republic, the city posadnik was electable out of boyars by veche, to be a mediator between the public and the Novgorod kniaz. This title was usually retained by the person after being replaced, with the addition of "old": "old posadnik", i.e., "ex-posadnik".
