Envelope

An envelope is a lightweight, flat package designed to enclose a letter or document for mailing or delivery. It is usually made of paper or another flexible material and has a sealable flap.

Envelopes are produced in a range of sizes and may include features such as a window to display an address, or a security tint or patterned interior to obscure contents. They are sealed by gummed moisture-activated adhesive, self-sealing strips, or peel-and-seal closures. Materials include plain paper, kraft, glassine, vellum, and plastic.

Common types include standard business envelopes, window envelopes, padded or bubble mailers for protection, and courier or document mailers. They are used to send letters, invoices, checks, legal notices, and other documents.

In mathematics, an envelope is a curve that is tangent to a family of curves, representing a boundary of a family of shapes. The word comes from French envelopper, to wrap or surround.