Real Dictionary

question mark
countable noun: A question mark is the punctuation mark ? which is used in writing at the end of a question.
countable noun: If there is doubt or uncertainty about something, you can say that there is a question mark over it.

question mark in American English
a mark of punctuation (?) put after a sentence, word, etc. to indicate a direct question, and also used to express doubt, uncertainty, etc.; interrogation mark
noun: a mark indicating a question: usually, as in English, the mark (?) placed after a question

question mark in British English
noun: the punctuation mark ?, used at the end of questions and in other contexts where doubt or ignorance is implied