This phrase is used to refer to an aircraft being taken to, and perhaps beyond, its designated altitude and speed limits. By extension, this phrase may be used to mean testing other limits, either within aerospace or in other fields. Sometimes referred to as "pushing out the envelope"(though this term seems to be used mostly in Great Britain).

Pushing the envelope has also become an expression in other fields, referring to the act of introducing new ideas in readily established concepts. An example would be a marketing campaign which advertised its product in a way never done before, or a video game which interacted with the player in a completely original fashion.

This phrase came into general use following the publication Tom Wolfe's book about the space programme, "The Right Stuff" (1979), where he writes, "One of the phrases that kept running through the conversation was ‘pushing the outside of the envelope’... [That] seemed to be the great challenge and satisfaction of flight test."

Wolfe didn't originate the term, although it's appropriate that he used it in a technical and engineering context, as it was first used in the field of mathematics.