'信封袋' (xìn fēng dài) literally combines '信' (letter), '封' (seal/envelope), and '袋' (bag/pouch). It refers to a type of envelope—specifically, a flat, sealed paper or plastic pouch used to enclose letters, documents, or small items for mailing or storage. Unlike standard envelopes (信封), which are typically folded and glued, 信封袋 often has a self-adhesive closure or is heat-sealed, and may be slightly thicker or more durable. It’s commonly used in office settings, schools, and postal services for sending formal correspondence, certificates, or sensitive materials.

This term emphasizes function and form: it’s not just any envelope, but one designed as a protective, ready-to-use pouch. You’ll see it on packaging labels, stationery catalogs, and logistics forms. While 信封 is the general word for 'envelope', 信封袋 highlights the container-like quality—especially useful when distinguishing between traditional folded envelopes and modern, pre-made pouch-style ones.