What is Ancient Demesne?

Legal Definition
Eng. law. Those lands which either were reserved to the crown at the original distribution of landed property, or such as came to it afterwards, by forfeiture or other means. 1. Sal. 57; hob. 88; 4 Inst. 264; 1 Bl. Com. 286; Bac. Ab. h. t.; F. N. B. 14.
-- Bouviers Law Dictionary
Legal Definition
Manors which in the time of William the Co.nqueror were in the hands of the crown, and are so recorded in the Domesday Book. Fitzh. Nat. Brev. 14. 56; Baker v. Wich, 1 Salk. 56. Tenure in ancient demesne may be pleaded in abatement to an action of ejectment. Rust v. Roe, 2 Burr. 1046. Also a species of copyhold, which differs, however, from common copyholds in certain privileges, but yet must be conveyed by surrender, according to the custom of the manor. There are three sorts:

(1) Where the lands are held freely by the king's grant;

(2) customary freeholds, which are held of a manor in ancient demesne, but not at the lord's will, although they are conveyed by surrender, or deed and admittance;

(3) lands held by copy of court-roll at the lord's will, denominated copyholds of base tenure.
-- Black's Law Dictionary
Legal Definition
A manor recorded in the Domesday Book as being in the hands of the crown during the reign of William the Conqueror.
-- Ballentine's Law Dictionary