Keywords: doubt, uncertainty, doubtful, unsure, uncertain
As a Noun
The feeling of uncertainty you have about something, especially wondering if it is true or false, good or bad, or possible or impossible. English = doubt, uncertainty.
As a Verb or Adjective
To feel some uncertainty about something, especially to wonder if it is true or false, good or bad, or possible or impossible. English = doubt; (be) doubtful, (be) unsure, (be) uncertain.
Used alone in response to a question to show your uncertainty and your desire to know more about the situation before you can give an answer. English = 'Not sure', 'It depends' and so on.