a flat paper container, as for a letter or thin package, usually having a gummed flap or other means of closure.

something that envelops; a wrapper, integument, or surrounding cover.

Biology . a surrounding or enclosing structure, as a corolla or an outer membrane.

Geometry . a curve or surface tangent to each member of a set of curves or surfaces.

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.

the fabric structure enclosing the gasbag of an aerostat.

the gasbag itself.

Electronics . the airtight glass or metal housing of a vacuum tube.

the technical limits within which an aircraft or electronic system may be safely operated. ?

push the envelope, to stretch established limits

as in technological advance or social innovation.

noun

bag , box , case , casing , cloak , coat , coating , container , cover , covering , enclosure , hide , jacket , pocket , pouch , receptacle , sheath , shell , skin , vesicle , wrappingnotes:envelop is the verb; envelope is the noun , capsule , inclosure , wrapper