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).
An uncontrolled or unpredictable element.
A character that takes the place of any other character or string that is not known or specified.
An element, often deliberately concealed, which is withheld for contingency.