noun

• (Ancient Greece, religion) The innermost sanctuary or shrine in a temple, from where oracles were given.
  "Let us now with minds free of paſſion, enter the adytum with an intent to find out its true figure, to examine what it really was, and what it is. […] This point is properly the door-way or entrance into the adytum, as a wicket or little door, whilſt the jambs of the hithermoſt trilithons preſent themſelves, as the greater door, of about 40 feet wide, 25 cubits. […] [T]he more ſacred part of the temple at Hierapolis anſwering to our Adytum, had no door, tho' none enter'd therein but the chief prieſts."
• (by extension) A private chamber; a sanctum.
  "Although it was the custom for the priestesses to affect a holy fury, a species of temporary insanity, in the delivering of their oracles, yet it was also the custom to write them on the leaves of trees, and to deposit them at the entrances of their caves, or the adyta of their temples; and it was the object of the devotee to secure them before they were dispersed by the winds; […]"