envelope

envelope
[ˈɛnvələʊp, ˈɒn-]
noun

a flat paper container with a sealable flap, used to enclose a letter or document
信封
a covering or containing structure or layer
封皮，封套；外表，外壳；包裹物
the external envelope of the swimming pool.
游泳池的外包层。
■the outer metal or glass housing of a vacuum tube, electric light, etc
（真空管或电灯等的）泡
■the structure within a balloon or non-rigid airship containing the gas
（气球或飞艇的）蒙皮，气囊
■(Microbiology)a membrane which forms the outer layer of certain viruses
(微生物)包膜，外膜，被膜
■(Electronics)a curve joining the successive peaks of a modulated wave
(电子)包络（曲）线
■(Mathematics)a curve or surface tangent to each of a family of curves or surfaces
(数)包络线（或面）

常用词组
the back of an envelope

used to denote calculations or plans of the most sketchy kind
粗略的估算（或计划）
a proposal drawn up on the back of an envelope.
一项粗略的提议。

push the envelope (或 the edge of the envelope)

(informal)approach or extend the limits of what is possible
(非正式)几近极致
these are extremely witty and clever stories that consistently push the envelope of TV comedy.
这些故事极其诙谐巧妙，总是将电视喜剧推向极致。

语源

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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