noun
General noun for a question or inquiry; used in both spoken and written Japanese.
先生に問いを投げかけた。
I posed a question to the teacher.
その問いには答えられない。
I cannot answer that question.
質問 is the most common word for 'question' in everyday conversation; 問い is slightly more formal or literary.
疑問 implies doubt or a problem to be solved, while 問い is a neutral question or inquiry.
Derived from the verb 問う (とう, 'to ask'), with the nominalizing suffix い.