Keywords: question mark, interrogation mark, query
As a Noun
A special punctuation mark which is put at the end of a written sentence. English = question mark, interrogation mark.
Something you check by asking about it because you are no sure that it is correct. English = query.
Used alone in response to your own or your addressee's previous comment to mean you wonder why such and such is the case. English = 'I wonder why?', 'Why's that?', and so on.
Used alone in response to your addressee failing to recognise that you have just asked a question to mean you want an answer. English = 'Well? Are you going to answer?', 'That was a question, you know?', 'Well, what's your answer?', and so on.
Used alone to dismiss your partner's question as unimportant or unanswerable. English = 'Who knows?', 'God knows!', 'I wouldn't have a clue!', and so on.