The innermost parts of a building; especially, of a temple or sanctuary: In Roman times, the penetralia were the parts of a Roman temple to which the priests alone had access, where the sacred images were kept, the responses of the oracles were made, and the sacred mysteries were performed.

The holy of holies was the penetralia of the Jewish Temple or the sanctuary that consisted of the innermost chamber of the Tabernacle in the temple of Solomon where the Ark of the Covenant was kept.