1. a flat paper container with a sealable flap, used to enclose a letter or document.
2. a covering or containing structure or layer: "the external envelope of the swimming pool"
▪ the outer metal or glass housing of a vacuum tube, electric light, etc.: "the switch itself is a glass envelope filled with rare gases and containing two contacts"
▪ the structure within a balloon or nonrigid airship containing the gas.
▪ a membrane forming the outer layer of certain viruses.
▪ a curve joining the successive peaks of a modulated wave.
▪ a curve or surface tangent to each of a family of curves or surfaces.
▪ the nebulous covering of the head of a comet; coma.
Word Originmid 16th century (in the sense ‘wrapper, enveloping layer’; originally as envelope): from French enveloppe, from envelopper ‘envelop’. The sense ‘covering of a letter’ dates from the early 18th century.