The Collaborative International Dictionary of English v.0.48:

Demesne \De*mesne"\, n. [OE. demeine, demain, rule, demesne, OF.
   demeine, demaine, demeigne, domaine, power, F. domaine
   domain, fr. L. dominium property, right of ownership, fr.
   dominus master, proprietor, owner. See Dame, and cf.
   Demain, Domain, Danger, Dungeon.] (Law)
   A lord's chief manor place, with that part of the lands
   belonging thereto which has not been granted out in tenancy;
   a house, and the land adjoining, kept for the proprietor's
   own use. [Written also demain.] --Wharton's Law Dict.
   Burrill.
   [1913 Webster]

   Ancient demesne. (Eng. Law) See under Ancient.
      [1913 Webster]

WordNet (r) 3.0 (2006):

demesne
    n 1: extensive landed property (especially in the country)
         retained by the owner for his own use; "the family owned a
         large estate on Long Island" [syn: estate, land,
         landed estate, acres, demesne]
    2: territory over which rule or control is exercised; "his
       domain extended into Europe"; "he made it the law of the
       land" [syn: domain, demesne, land]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "demesne":
   acres, ambit, arable land, area, arena, bailiwick, barnyard,
   barton, beat, border, borderland, cattle ranch, champaign,
   chattels real, chicken farm, circle, circuit, collective farm,
   cotton plantation, croft, dairy farm, demesne farm, department,
   discipline, domain, dominion, dry farm, dude ranch, factory farm,
   fallow, farm, farmery, farmhold, farmland, farmplace, farmstead,
   farmyard, field, fruit farm, fur farm, grain farm, grange,
   grassland, grounds, hacienda, hemisphere, homecroft, homefarm,
   homestead, honor, judicial circuit, jurisdiction, kibbutz, kolkhoz,
   land, landed property, lands, location, lot, lots, mains, manor,
   manor farm, march, messuage, orb, orbit, orchard, pale, parcel,
   pasture, pen, plantation, plat, plot, poultry farm, praedium,
   precinct, property, province, quadrat, ranch, rancheria, rancho,
   real estate, real property, realm, realty, round, sheep farm,
   sphere, station, steading, stock farm, subdiscipline, tenements,
   terrain, territory, toft, truck farm, walk



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEMESNE, Eng. law. The name given to that portion of the lands of a manor
which the lord retained in his own hands for the use of himself and family.
These lands were called terra dominicales or demesne lands, because they
were occupied by the lord, or dominus manerii, and his servants, &c. 2 Bl.
Com. 90. Vide Ancient Demesne; Demesne as of fee; and Soil assault demesne.