noun.
A riddle in which a fanciful question is answered by a pun.
noun.
A paradoxical, insoluble, or difficult problem; a dilemma.
The Century Dictionary and Cyclopedia
noun.
A conceit; a device; a hoax.
noun.
A riddle in which some odd resemblance is proposed for discovery between things quite unlike, or some odd difference between similar things, the answer often involving a pun.
the GNU version of the Collaborative International Dictionary of English
noun.
A kind of riddle based upon some fanciful or fantastic resemblance between things quite unlike; a puzzling question, of which the answer is or involves a pun.
noun.
A question to which only a conjectural answer can be made.