A feeling or state of uncertainty about the truth, correctness, or appropriateness of something. Used both as a noun and as a na-adjective (疑問な, 疑問に思う).
His explanation leaves some questions unanswered.
I felt doubtful about the plan.
Her behavior is questionable.
I still have doubts about his explanation.
The teacher carefully answered the students' questions.
Reading the document resolved a doubt I'd had for years.
The survey results raised doubts about the conventional theory.
疑問 refers to a doubt or unspoken question in one's mind, whereas 質疑 is a formally voiced inquiry addressed to someone.
疑問 is a specific question or doubt one has, while 謎 is a broader unsolved puzzle or enigma.
疑問 focuses on a feeling of doubt or uncertainty, while 問題 (in this sense) is the specific point being questioned.
disputed ballot; questioned ballot
doubt; question; be skeptical of