The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

envelope
    n 1: a flat (usually rectangular) container for a letter, thin
         package, etc.
    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 (especially an
       aircraft); "test pilots try to push the envelope"
    6: the bag containing the gas in a balloon [syn: envelope,
       gasbag]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "envelope":
   aura, bandage, bandaging, binder, binding, blaze of glory, border,
   brilliance, brilliancy, charisma, circumference, cortex, covering,
   crust, dust jacket, envelopment, epidermis, exterior, external,
   facade, face, facet, fringe, front, gift wrapping, glamour, glory,
   halo, illustriousness, integument, jacket, lineaments, luster,
   magic, mystique, nimbus, numinousness, outer face, outer layer,
   outer side, outer skin, outline, outside, periphery, radiance,
   resplendence, resplendency, rind, shell, skin, splendor,
   superficies, superstratum, surface, top, wrap, wrapper, wrapping



The Devil's Dictionary (1881-1906):

ENVELOPE, n.  The coffin of a document; the scabbard of a bill; the
husk of a remittance; the bed-gown of a love-letter.