The Collaborative International Dictionary of English v.0.48:

Realm \Realm\ (r[e^]lm), n. [OE. realme, ream, reaume, OF.
   reialme, roialme, F. royaume, fr. (assumed) LL. regalimen,
   from L. regalis royal. See Regal.]
   1. A royal jurisdiction or domain; a region which is under
      the dominion of a king; a kingdom.
      [1913 Webster]

            The absolute master of realms on which the sun
            perpetually shone.                    --Motley.
      [1913 Webster]

   2. Hence, in general, province; region; country; domain;
      department; division; as, the realm of fancy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

realm
    n 1: a domain in which something is dominant; "the untroubled
         kingdom of reason"; "a land of make-believe"; "the rise of
         the realm of cotton in the south" [syn: kingdom, land,
         realm]
    2: the domain ruled by a king or queen [syn: kingdom, realm]
    3: a knowledge domain that you are interested in or are
       communicating about; "it was a limited realm of discourse";
       "here we enter the region of opinion"; "the realm of the
       occult" [syn: region, realm]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "realm":
   ally, ambit, animal kingdom, archduchy, archdukedom, area, arena,
   bailiwick, beat, body politic, border, borderland, buffer state,
   captive nation, chain of being, chieftaincy, chieftainry, circle,
   circuit, city-state, class structure, colony, commonweal,
   commonwealth, compass, confines, country, county, demesne,
   department, dimensions, discipline, domain, dominion, duchy,
   dukedom, earldom, empery, empire, establishment, extent, field,
   free city, grand duchy, hemisphere, hierarchy, judicial circuit,
   jurisdiction, kingdom, land, limits, mandant, mandate,
   mandated territory, mandatee, mandatory, march, mineral kingdom,
   monarchy, nation, nationality, natural hierarchy, orb, orbit, pale,
   pecking order, polis, polity, possession, power, power structure,
   precinct, principality, principate, protectorate, province,
   puppet government, puppet regime, purview, pyramid, radius, reach,
   republic, responsibility, round, satellite, scope, seneschalty,
   settlement, sovereign nation, sphere, state, subdiscipline,
   sultanate, superpower, sweep, territory, toparchia, toparchy,
   vegetable kingdom, walk



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

REALM. A kingdom; a country. 1 Taunt. 270; 4 Campb. 289; Rose, R. 387.