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.
Idioms
push the envelope, to stretch established limits, as in technological advance or social innovation.
Exceed the limits of what is normally done, be innovative, as in They are pushing the envelope in using only new fabrics for winter clothing. This idiom comes from aviation, the envelope alluding to the technical limits of a plane's performance, which, on a graph, appear as a rising slope as limits of speed and stress are approached and falls off when the capacity is exceeded and the pilot loses control; safety lies within these limits, or envelope, and exceeding them exposes pilot and plane to risk. [Slang; late 1960s]
