An envelope is a flat, usually rectangular or square paper container used to hold letters, documents, or cards. It typically has a sealable flap that can be closed to secure the contents inside. Envelopes come in various sizes and colors, making them suitable for different purposes, such as sending invitations, bills, or personal correspondence. Envelopes are commonly used in mailing systems, where they protect the contents during transit. They can be addressed with the recipient's name and address, and often have a return address for the sender. Many envelopes are designed to be compatible with postal services, ensuring safe delivery to the intended recipient.