Duchy

A Duchy is simpliciter the territory under the authority of a Duke.

Given that dukes were, in effect, second only to a monarch, duchies in practice tended to be given a name attached to a prominent estate, but were made up of parcels of land spread throughout a realm, so as to prevent the undue imbalancing of power between monarch and subject.

Again, several period duchies had broken away from their nominal superiors and were, in all effects, miniature kingdoms of their own -- an example lies in the Duchy of Brittany, which for periods managed to hover between the French and English crowns, overtly owing allegiance to both and neither (i.e. always telling the one that their liege lay in the other crown), and functioned as an independent entity.