envelope
/ˈenvələʊp/noun
1
- a flat paper container with a sealable flap, used to enclose a letter or document信封。
2
- 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.
游泳池的外罩。
2.1
- the outer metal or glass housing of a vacuum tube, electric light, etc.（真空管或电灯等的）泡。
2.2
- the structure within a balloon or non-rigid airship containing the gas（气球或软式飞艇的）气囊。
2.3
- Microbiology a membrane which forms the outer layer of certain viruses【微生物】包膜, 外膜, 被膜。
2.4
- Electronics a curve joining the successive peaks of a modulated wave【电子】包络（曲）线。
2.5
- Mathematics a curve or surface tangent to each of a family of curves or surfaces【数】包络线（或面）。
短语
the back of an envelope
- used to denote calculations or plans of the most sketchy kind（指估计或计划）最粗略的一类：
a proposal drawn up on the back of an envelope.
一项草拟的粗略提议。
push the envelope（或 the edge of the envelope）
- informal approach or extend the limits of what is possible〈非正式〉接近（或达到）极致：
these are extremely witty and clever stories that consistently push the envelope of TV comedy.
这些故事极其诙谐和巧妙, 总是将电视喜剧推向极致。
ORIGIN: originally aviation slang, relating to graphs of aerodynamic performance.
词源
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.