envelope

{ } n. [ F. enveloppe. ] 1. That which envelops, wraps up, encases, or surrounds; a wrapper; an inclosing cover; esp., the cover or wrapper of a document, as of a letter. [ 1913 Webster ]

2. (Astron.) The nebulous covering of the head or nucleus of a comet; -- called also coma. [ 1913 Webster ]

3. (Fort.) A work of earth, in the form of a single parapet or of a small rampart. It is sometimes raised in the ditch and sometimes beyond it. Wilhelm. [ 1913 Webster ]

4. (Geom.) A curve or surface which is tangent to each member of a system of curves or surfaces, the form and position of the members of the system being allowed to vary according to some continuous law. Thus, any curve is the envelope of its tangents.

4. A set of limits for the performance capabilities of some type of machine, originally used to refer to aircraft; -- it is often described graphically as a two-dimensional graph of a function showing the maximum of one performance variable as a function of another. Now it is also used metaphorically to refer to capabilities of any system in general, including human organizations, esp. in the phrase push the envelope. It is used to refer to the maximum performance available at the current state of the technology, and therefore refers to a class of machines in general, not a specific machine. [ PJC ]

push the envelope to increase the capability of some type of machine or system; -- usually by technological development. [ 1913 Webster ]