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The Collaborative International Dictionary of English v.0.48 (gcide)

Envelope \En"vel*ope\ (?; 277), Envelop \En*vel"op\ (?; 277), 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]