理由 (riyū) means 'reason.' It refers to the cause or explanation for something. When you ask 'なぜ？' (Why?), the answer will often include 理由. For example, '遅れた理由を教えてください' (Please tell me the reason you were late).

「理由」は、物事の原因や根拠のことです。例：「理由を説明する」