enigma
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1. a. A short composition in prose or verse, in which something is
described by intentionally obscure metaphors, in order to afford an
exercise for the ingenuity of the reader or hearer in guessing what is
meant; a riddle.    {dag}b. In wider sense: An obscure or allusive
speech; a parable (obs. exc. as transf. from 1a).

    2. fig. Something as puzzling as an enigma; an unsolved problem.