envelope

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.

Something that envelops; a wrapping.

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.

A mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.

A curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.

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.

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.