Today's lesson: The phrase "pushing the envelope" comes from aviation slang. In mathematics, an envelope is a curve that you get when you connect the successive peaks of a wave. In aeronautics, the term envelope refers to the known limits for the safe performance of an airplane. To push the envelope means to test the limits of what the plane is capable of doing. Tom Wolfe used the phrase in his 1979 book, "The Right Stuff," but it wasn't used outside of an aviation context until the late 1980s.