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1a flat paper container with a sealable flap, used to enclose a letter or document. 2a 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 structure within a balloon or nonrigid airship containing the gas. Microbiology a membrane forming the outer layer of certain viruses. Electronics a curve joining the successive peaks of a modulated wave. Mathematics a curve or surface tangent to each of a family of curves or surfaces. Astronomy the nebulous covering of the head of a comet; coma.