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Pushing the Envelope
This is an aviation term. It means pushing the aircraft to its limits, especially in the context of flight testing. Envelope has several secondary definitions referring to a collection of curves (mathematical and engineering jargon). So in the world of aeronautical engineering the envelope is the collection of curves that describe the maximum performance of an aircraft. To push the envelope is to take the aircraft to the edge of what it was designed to do and try and take it beyond.