"When it comes to testing new aircraft or determining maximum performance, pilots like to talk about 'pushing the envelope.' They're talking about a two dimensional model: the bottom is zero altitude, the ground; the left is zero speed; the top is max altitude; and the right, maximum velocity, of course. So, the pilots are pushing that upper-right-hand corner of the envelope. What everybody tries not to dwell on is that that's where the postage gets canceled, too."

— Admiral Rick Hunter, U.S. Navy.

Quoted in skygod.com