One of the phrases that kept running through their — Tom Wolfe

One of the phrases that kept running through their conversation was “pushing the outside of the envelope.” The “envelope” was a flight-test term referring to the limits of a particular aircraft’s performance, how tight a turn it could make at such-and-such a speed, and so on. “Pushing the outside,” probing the outer limits, of the envelope seemed to be the great challenge and satisfaction of flight test.