The Collaborative International Dictionary of English v.0.48:

Adytum \Ad"y*tum\, n. Adyta. [L., fr. Gr. ?, n., fr. ?, a.,
   not to be entered; 'a priv. + ? to enter.]
   The innermost sanctuary or shrine in ancient temples, whence
   oracles were given. Hence: A private chamber; a sanctum.
   [1913 Webster] Adz