谜 (mí): énigme

On the evening of the Lantern Festival, the square was decorated with red lanterns, and under each lantern was written a riddle; those who guessed correctly could get a small gift.

This math problem is too difficult; I worked on it for half an hour and still couldn't solve it. To me, it's simply a riddle.

Why he suddenly resigned, nobody knows the reason; this matter became a riddle in the company, and colleagues were all guessing privately.

Grandpa likes playing riddle-guessing games with his grandson; he poses a riddle, and the grandson often thinks for a long time and still can't guess the answer.

This novel leaves a riddle right from the start: the protagonist wakes up and finds himself in a strange room, with no memory of who he is.