An envelope is the rectangular paper cover in which you send a letter to someone through the post.
PHRASE
挑戰(zhàn)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(shí)尚和設(shè)計(jì)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(zhàn)家庭娛樂片的接受程度方面，沒有哪個(gè)電影導(dǎo)演比他的影響更深遠(yuǎn)。