Meaning of question mark

ques'tion mark"

Pronunciation: [key]
  1. Also calleda 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.