en·ve·lope 
/ˈɛnvəˌloʊp, ˈɑ:nvəˌloʊp/ noun
plural en·ve·lopes /ˈɛnvəˌloʊp, ˈɑ:nvəˌloʊp/ noun [count] : an enclosing cover for a letter, card, etc. — see picture at mail
push the envelope
: to go beyond the usual or normal limits by doing something new, dangerous, etc.
▪ a director who has pushed the envelope in his recent films ▪ a new airplane design that pushes the envelope
▪ a director who has pushed the envelope in his recent films ▪ a new airplane design that pushes the envelope
