The head of a mafia crime family, usually
Italian, ex. Don Corleone from "The Godfather".
The Don of a mafia is the boss of the family and also of the family business. He can declare war on another mafia family and he can also call and make a truce or surrender regarding such family wars. He can also arrange marriages of duaghters from the family even if the daughter is not his own. If you are
late to a family meeting with the Don you better be wearing a
bullet-proof vest.