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Definition of envelope in English   Translate envelope | into
French | into German | into Italian | into Spanish     • 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 switch itself is a glass envelope filled with rare gases and
containing two contacts   • the structure within a balloon or
non-rigid airship containing the gas.   • Microbiology a
membrane which forms 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.   the back of an envelope  used in
reference to calculations or plans of the most sketchy kind:a
proposal drawn up on the back of an envelope  push the envelope
(or the edge of the envelope