1
general::
Exceed the limits of what is normally done, be innovative, as in They are pushing the envelope in using only new fabrics for winter clothing. This idiom comes from aviation, the envelope alluding to the technical limits of a plane's performance, which, on a graph, appear as a rising slope as limits of speed and stress are approached and falls off when the capacity is exceeded and the pilot loses control; safety lies within these limits, or envelope, and exceeding them exposes pilot and plane to risk. [Slang; late 1960s]
American Heritage Idioms
2
general::
Phrase(s): push the envelope
Fig. to expand the definition, categorization, dimensions, or perimeters of something. • The engineers wanted to completely redesign the product, but couldn’t push the envelope because of a very restricted budget.
McGrawhill's American Idioms And Phrasal Verbs