An envelope is a flat, usually rectangular or square paper container used to enclose a letter or other documents. It typically has a sealable flap that can be closed to secure the contents inside. Envelopes come in various sizes and colors, and they are commonly used for mailing letters, invitations, and greeting cards. Envelopes are often made from materials like paper or cardstock and can be designed for specific purposes, such as business correspondence or personal communication. They may also feature printed designs or logos, making them suitable for branding in marketing materials.