an old legal term, not limited to lands. Per Black's Law Dictionary:

"Demesne: Domain; dominical; held in one's own right, and not of a superior; not allotted to tenants."

"Demesne Lands: In English law. Those lands of a manor not granted out in tenancy, but reserved by the lord for his own use and occupation. Lands set apart and appropriated by the lord for his own private use, as for the supply of his table, and the maintenance of his family; the opposite of *tenemental lands."

and the historical import:

Ancient Demesne: Manors which in the time of Willliam the Conqueror were in the hands of the crown, and are so recorded in the Domesday Book.