[en-vuh-lohp, ahn-]
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.
push the envelope, to stretch established limits, as in technological advance or social innovation.
Also, envelop.

1700–10; < French enveloppe, derivative of envelopper to envelop

envelop, envelope.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
envelope (ˈɛnvəˌləʊp, ˈɒn-)
1.  a flat covering of paper, usually rectangular in shape and with a flap that can be folded over and sealed, used to enclose a letter, etc
2.  any covering or wrapper
3.  biology any enclosing structure, such as a membrane, shell, or skin
4.  the bag enclosing the gas in a balloon
5.  maths a curve or surface that is tangent to each one of a group of curves or surfaces
6.  electronics the sealed glass or metal housing of a valve, electric light, etc
7.  telecomm the outer shape of a modulated wave, formed by the peaks of successive cycles of the carrier wave
8.  informal push the envelope to push the boundaries of what is possible
[C18: from French enveloppe, from envelopper to wrap around; see envelop; sense 8 from aeronautics jargon, referring to graphs of aircraft performance]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1705, from Fr. enveloppe, a back-formation of envelopper "to envelop" (see envelop).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

envelope en·ve·lope (ěn'və-lōp', ŏn'-)
An enclosing structure or cover, such as a membrane or the outer coat of a virus.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
One thing that separates mere stamps from postal history is the evocativeness
  of an envelope or postcard.
Please include a self-addressed envelope for return of work samples.
Some of those who are thought to be listed in the sealed envelope have keenly
  endorsed the idea of a tribunal.
Hold the guide envelope firmly in the center with one hand, and use a pencil or
  pen to trace around it.
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