Envelope

• An envelope is a sheet of paper given a specific shape (typically that of a rhombus or a short arm cross) which ensures that when its sides are folded about a central rectangular area a rectangular-faced enclosure is formed with an arrangement of four flaps on the reverse side with overlapping edges; although in principle the flaps can be held in place by securing the topmost flap at a single point(for example with a wax seal), generally they are pasted or gummed together at the overlaps. They are most commonly used for enclosing and sending mail (letters) through a prepaid-postage postal system.As they are made of paper they are intrinsically amenable to embellishment with additional graphics and text over and above the necessary postal indicia. This is a feature which the direct mail industry has long taken advantage of, and more recently the Mail Art movement.
• An envelope is a set of headers for an e-mail
• An envelope is a mathematical curve defined as being tangent to a given family of lines (or curves).
• An envelope is a curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
• An envelope is the shape of a sound which may be controlled by a synthesizer or sampler in music, derived from the above definition for electronics.