An envelope is the rectangular paper cover in which you send a letter to someone through the post.
PHRASE
挑戰限制尺度；突破原有底線
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時尚和設計受到人們的重視。No cinema director had a more profound effect in pushing the envelope of acceptable family entertainment.在挑戰家庭娛樂片的接受程度方面，沒有哪個電影導演比他的影響更深遠。