The Collaborative International Dictionary of English v.0.48:

Sanctum \Sanc"tum\, n. [L., p. p. of sancire to consecrate.]
   A sacred place; hence, a place of retreat; a room reserved
   for personal use; as, an editor's sanctum.
   [1913 Webster]

   Sanctum sanctorum [L.], the Holy of Holies; the most holy
      place, as in the Jewish temple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sanctum
    n 1: a place of inviolable privacy; "he withdrew to his sanctum
         sanctorum, where the children could never go" [syn:
         sanctum, sanctum sanctorum]
    2: a sacred place of pilgrimage [syn: holy place, sanctum,
       holy]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "sanctum":
   adytum, ashram, cabinet, cell, cloister, closet, den, hermitage,
   hideaway, hideout, hiding place, holy of holies, holy place,
   ivory tower, lair, mew, privacy, recess, retreat, sacrarium,
   sanctuary, sanctum sanctorum, secret place, shrine, study