envelope

noun

1. a flat paper container, as for a letter or thin package, usually having a gummed flap or other means of closure.
2. something that envelops; a wrapper, integument, or surrounding cover.
3. Biology. a surrounding or enclosing structure, as a corolla or an outer membrane.
4. Geometry. a curve or surface tangent to each member of a set of curves or surfaces.
5. Radio. (of a modulated carrier wave) a curve connecting the peaks of a graph of the instantaneous value of the electric or magnetic component of the carrier wave as a function of time.
6. the fabric structure enclosing the gasbag of an aerostat.
7. the gasbag itself.
8. Electronics. the airtight glass or metal housing of a vacuum tube.
9. the technical limits within which an aircraft or electronic system may be safely operated.

noun

1. a flat covering of paper, usually rectangular in shape and with a flap that can be folded over and sealed, used to enclose a letter, etc
2. any covering or wrapper
3. biology any enclosing structure, such as a membrane, shell, or skin
4. the bag enclosing the gas in a balloon
5. maths a curve or surface that is tangent to each one of a group of curves or surfaces
6. electronics the sealed glass or metal housing of a valve, electric light, etc
7. telecomm the outer shape of a modulated wave, formed by the peaks of successive cycles of the carrier wave
8. push the envelope informal to push the boundaries of what is possible

n.

1. An enclosing structure or cover, such as a membrane or the outer coat of a virus.