All 4 Uses

enigma
in
The Gold-Bug
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Were all the jewels of Golconda awaiting me upon my solution of this enigma, I am quite sure that I should be unable to earn them.†
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Circumstances, and a certain bias of mind, have led me to take interest in such riddles, and it may well be doubted whether human ingenuity can construct an enigma of the kind which human ingenuity may not, by proper application, resolve.†
"But," said I, "the enigma seems still in as bad a condition as ever.†
Upon this discovery I was so sanguine as to consider the enigma solved; for the phrase 'main branch, seventh limb, east side,' could refer only to the position of the skull upon the tree, while 'shoot from the left eye of the death's head' admitted, also, of but one interpretation, in regard to a search for buried treasure.†

Definitions:

(1) (enigma)
something mysterious that seems unexplainable
(2) (meaning too rare to warrant focus)
The word may also be seen in the proper noun, Enigma Machine. It refers to an encryption device used by Nazi Germany during the second world war. Britain eventually learned to unencrypt messages sent with the machine, but kept that ability a secret.