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.

    e.g. There's a valuable place for fashion and design that pushes the envelope a bit...
           在原有尺度上有所突破的时尚和设计受到人们的重视。
    e.g. No cinema director had a more profound effect in pushing the envelope of acceptable family entertainment.
           在挑战家庭娱乐片的接受程度方面，没有哪个电影导演比他的影响更深远。