Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. (Astron.) The nebulous covering of the head or nucleus of
      a comet; -- called also {coma}.

   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.

   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.

Source : WordNet®

envelope
     n 1: a flat rectangular paper container for papers
     2: any wrapper or covering
     3: a curve that is tangent to each of a family of curves
     4: a natural covering (as by a fluid); "the spacecraft detected
        an envelope of gas around the comet"
     5: the maximum operating capability of a system; "test pilots
        try to push the envelope"
     6: the bag containing the gas in a balloon [syn: {gasbag}]