An envelope is a flat, usually rectangular or square paper container used to hold letters, cards, or other documents. It typically has a sealable flap that can be closed to secure the contents inside. Envelopes come in various sizes, colors, and materials, making them suitable for different purposes, such as mailing or storing important papers. Envelopes are commonly used in everyday communication, especially for sending physical mail through postal services like the United States Postal Service. They can also be used for invitations, greeting cards, and business correspondence, providing a professional appearance and protecting the contents from damage.