envelope /ˈɛnvələʊp, ˈɒnvələʊp/

e

I. noun

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.
3. 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.
4. the structure within a balloon or non-rigid airship containing the gas.
5.
[Microbiology] a membrane which forms the outer layer of certain viruses.
6.
[Electronics] a curve joining the successive peaks of a modulated wave.
7.
[Mathematics] a curve or surface tangent to each of a family of curves or surfaces.
II. phrases

1. 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.
2. push the envelope (or the edge of the envelope)
approach or extend the limits of what is possible

these are extremely witty and clever stories that consistently push the envelope of TV comedy.
[originally aviation slang, relating to graphs of aerodynamic performance.]
– origin mid 16th cent. (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 cent.

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