Rev. 17:5

And upon her forehead was a name written,
MYSTERY,
  BABYLON THE GREAT, THE MOTHER OF HARLOTS
             AND ABOMINATIONS OF THE EARTH.


- A
mystery in Scripture is a previously hidden truth, now
divinely revealed...                                         (Scofield)

- Babylon: The most corrupt seat of idolatry and the enemy
of Christianity.                                                (Thayer's Lexicon)

- Harlot: One who yields herself to defilement... whether for
gain or lust; a fornicator...                           (Thayer's Lexicon)


Combine the three... then, what is Mystery Babylon?