enigma

Definitions:

A purposely obscure saying or question propounded for solution; a riddle; any puzzling or unaccountable phenomenon or proceeding.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
A riddle; a dark saying; anything obscure or ambiguous.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
Enigmatic, enigmatical.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)
A riddle; mystery.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)
A riddle; anything that puzzles or baffles.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
A statement with a hidden meaning to be guessed: anything very obscure: a riddle.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)

Usage examples:

While the man thus spoke, the duke looked fixedly at him in perfect silence, in the forlorn hope of hearing some solution to the enigma
- The Lost Lady of Lone by E.D.E.N. Southworth
The countess answered with a smile that she was not an enigma for everybody, although she was necessarily so for most people.
- The Memoires of Casanova, Complete The Rare Unabridged London Edition Of 1894, plus An Unpublished Chapter of History, By Arthur Symons by Jacques Casanova de Seingalt
She was an enigma to- day- pale, brooding, silent- and she never looked at me except when my face was half averted.
- The Rustlers of Pecos County by Zane Grey