信件 (xìn jiàn) is a formal, written noun for 'letter' or 'mail' as a category, often used in official or general contexts. It is more formal than 信 (xìn), which is the everyday spoken word for a single letter or mail. Unlike 启事 (qǐ shì), which is a public notice or ad, 信件 refers specifically to personal or business correspondence sent by post.