An enigmatic, inscrutable, or mysterious person or thing; an enigma, a riddle.
The punctuation mark "?", used at the end of a sentence to indicate a question.
A state of doubt or uncertainty.
Polygonia interrogationis, a North American nymphalid butterfly with a silver mark on the underside of its hindwing resembling a question mark (sense 1).
A person or thing whose nature or value is a mystery.
Used other than figuratively or idiomatically: see unknown, quantity.