1. Also called interrogation point, interrogation mark. a mark indicating a question: usually, as in English, the mark (?) placed after a question.
2. something unanswered or unknown: His identity is still a question mark to most of us.
3. an anglewing butterfly, Polygonia interrogationis, having silver spots shaped like a question mark on the underside of each hind wing.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.