Thayer's Greek Lexicon

 1. hidden thing, secret, mystery
  1) generally mysteries, religious secrets, confided only to the
initiated and not to ordinary mortals
  2) a hidden or secret thing, not obvious to the understanding
  3) a hidden purpose or counsel
1c1. secret will
1c  1) of men
1c  2) of God: the secret counsels which govern God in
dealing with the righteous, which are hidden from
ungodly and wicked men but plain to the godly
3. in rabbinic writings, it denotes the mystic or hidden sense:
of an OT saying, of an image or form seen in a vision, of a dream