An envelope is the rectangular paper cover in which you send a letter to someone through the post.
PHRASE
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.
例句
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。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。