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.

A question to which only a conjectural answer can be made.

a puzzle, a riddle, a mystery, a question

Conundrum is an original novel written by Steve Lyons and based on the long-running British science fiction television series Doctor Who. It features the Seventh Doctor, Ace and Bernice. A prelude to the novel, also penned by Lyons, appeared in Doctor Who Magazine #208.

Conundrum is a Dragonlance fantasy novel by Jeff Crook.

Crystalline reeds, strands or shards; the shattered physical form of the Sanguinary.

"Conundrum" is the title of a Star Trek: The Next Generation episode, from the fifth season. Its production number is 214, and it first aired on 17 February, 1992.

a difficult problem

(noun) An intricate and difficult problem.