疑问 (yíwèn) is a noun meaning 'doubt' or 'question' — the feeling of uncertainty or a point you want clarified. Unlike 问题 (wèntí 'problem, issue'), 疑问 focuses on uncertainty rather than a difficulty. It differs from verbs like 怀疑 (huáiyí 'to suspect') and 疑惑 (yíhuò 'to be puzzled'), which are actions or states; 疑问 is the noun form for the doubt itself.