envelope

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A flat paper container, especially for a letter, usually having a gummed flap.
2. n. Something that envelops; a wrapping.
3. n. Biology An enclosing structure or cover, such as a membrane or the outer coat of a virus.
4. n. The bag containing the gas in a balloon or airship.
5. n. The set of limitations within which a technological system, especially an aircraft, can perform safely and effectively.
6. n. The coma of a comet.
7. n. Mathematics A curve or surface that is tangent to every one of a family of curves or surfaces.
8. idiom. push the envelope To increase the operating capabilities of a technological system.
9. idiom. push the envelope To exceed the existing limits in a certain field; be innovative.

Wiktionary

1. n. A paper or cardboard wrapper used to enclose small, flat items, especially letters, for mailing.
2. n. Something that envelops; a wrapping
3. n. A bag containing the lifting gas of a balloon or airship; fabric that encloses the gas-bags of an airship.
4. n. geometry 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. n. electronics A curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
6. n. music The shape of a sound, which may be controlled by a synthesizer or sampler.
7. n. computing The information used for routing an email that is transmitted with the email but not part of its contents.
8. n. biology An enclosing structure or cover, such as a membrane.
9. n. engineering The set of limitations within which a technological system can perform safely and effectively.
10. n. The nebulous covering of the head or nucleus of a comet; a coma.
11. n. An earthwork in the form of single parapet or a small rampart, sometimes raised in the ditch and sometimes beyond it.
12. v. nonstandard alternative spelling of envelop.

GNU Webster's 1913

1. n. 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. n. (Astron.) The nebulous covering of the head or nucleus of a comet; -- called also coma.
3. n. (Fort.) A work of earth, in the form of single parapet or of a small rampart. It is sometimes raised in the ditch and sometimes beyond it.
4. n. (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.
5. n. 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.

WordNet 3.0

1. n. a flat (usually rectangular) container for a letter, thin package, etc.
2. n. any wrapper or covering
3. n. a natural covering (as by a fluid)
4. n. the bag containing the gas in a balloon
5. n. the maximum operating capability of a system (especially an aircraft)
6. n. a curve that is tangent to each of a family of curves