a flat container, usu of folded and gummed paper, e.g. for a letter
a wrapper, cover, or casing, e.g. the gasbag of an airship
something that envelops; a wrapper or covering
an enclosing membrane or other natural covering that encloses
in mathematics, a curve tangent to each of a family of curves
push the (edge of) the envelope
to achieve success in a pioneering activity
[from graphs showing the performance capacity of a machine, aircraft, etc; the envelope is a line at the edge of the graph marking its limits][French enveloppe from Old French envoloper: see envelop]