An ELT Glossary : Open Questions / Wh-questions
Open questions are those which cannot be answered by either yes or no (see Closed Questions), but start with a question word like Where, Whereabouts, When, What, Who, How, How often, and require a response containing unknown, and unpredictable information (even if in some circumstances the asker may have a good idea of the most likely reply). As an example, the question When are you going back to London? could be answered by any of Next Tuesday / I haven't decided yet / On September 7th / I've decided not to go etc etc
Because most question words begin with Wh- in English, they are often also called Wh-Questions.