名词

1. 信封
An envelope is the rectangular paper cover in which you send a letter to someone through the post.

2. 挑战限制尺度；突破原有底线
If someone pushes the envelope, they do something to a greater degree or in a more extreme way than it has ever been done before.

1. any wrapper or covering

2. a flat (usually rectangular) container for a letter, thin package, etc.

3. the bag containing the gas in a balloon

Synonym: gasbag

4. the maximum operating capability of a system (especially an aircraft)

e.g. test pilots try to push the envelope

5. a natural covering (as by a fluid)

e.g. the spacecraft detected an envelope of gas around the comet

6. a curve that is tangent to each of a family of curves