envelope

noun

1. A paper or cardboard wrapper used to enclose small, flat items, especially letters, for mailing.

   “Now we are liberal with our innermost secrets, spraying them into the public ether with a generosity our forebears could not have imagined. Where we once sent love letters in a sealed envelope, or stuck photographs of our children in a family album, now such private material is despatched to servers and clouds operated by people we don't know and will never meet.”

2. Something that envelops; a wrapping.
3. A bag containing the lifting gas of a balloon or airship; fabric that encloses the gas-bags of an airship.

   “They have no internal or external support structure, being simply a fabric bag (or envelope) filled with a lighter than air gas. Inside the envelope are one or more "ballonets", or smaller bags, which help maintain the envelope's shape.”

4. A mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.
5. A curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
6. The shape of a sound, which may be controlled by a synthesizer or sampler.
7. The information used for routing a message that is transmitted with the message but not part of its contents.
8. An enclosing structure or cover, such as a membrane; a space between two membranes
9. The set of limitations within which a technological system can perform safely and effectively.

   “push the envelope”

   “Few of the lads had ever been in combat and they knew little about the critical tolerances of fighter aircraft during violent maneuvers. They knew where the outside of the envelope was, but they didn't know about the part where you reached the outside and then stretched her a little . . .”

10. The nebulous covering of the head or nucleus of a comet; a coma.
11. An earthwork in the form of a single parapet or a small rampart, sometimes raised in the ditch and sometimes beyond it.

    “make a blind all along the bottom of the ditch of the Envelope”

verb

1. (rare, transitive)To put (something) in an envelope.

   “Arthur Armytage drew the precious document from his bureau; and without trusting himself to a re-perusal, enveloped and re-enveloped—sealed and resealed it;—mounted his horse, and rode off to Greta Castle.”

   “This business of returning letters to the writers gives occupation to about sixty lady clerks in the “Return-Letter Department,” in the gallery up-stairs. To them belong the duties of reënveloping the letters in the well-known, yellow, dead-letter envelope, by which, under an act of Congress approved June 12, 1866, these are returned to the writers free of charge.”

   “I suspect you write letters as hens lay eggs, find that Lady Hamilton finds them, envelopes them, puts them before you as official letters, and you direct them as per memorandum affixed.”

   “It—the letter—was only an expression of the sender’s ardent desire to lend him, if not a minor, ten thousand pounds on his own security in the strictest confidence. To play his part out, he re-enveloped and pocketed it.”

   “Successes such as these have paved the way for our most recent marketing innovation. I refer to Canada Post edict No. 58291, stating that we will no longer redirect first-class mail without charge when households move. Ordering the new householder to re-envelope the mail and pay another 43 cents was a stroke of genius. New revenue potential is substantial — nearly 400,000 Canadians move to another province in any year and far more move within their province.”

2. (alt-of, archaic)Archaic form of envelop.

   “Again, if the plane of the impressed couple intersects the mean plane between N and C, it will envelope the cone whose focals are ON, ON′, and whose internal axis is therefore OA.”