en·ve·lope 
(ĕnvə-lōp′, ŏn-)
(ĕnvə-lōp′, ŏn-)n.
Idiom: 1. A flat paper container, especially for a letter, usually having a gummed flap.
2. Something that envelops; a wrapping.
3. Biology An enclosing structure or cover, such as a membrane or the outer coat of a virus.
4. The bag containing the gas in a balloon or airship.
5. The set of limitations within which a technological system, especially an aircraft, can perform safely and effectively.
6. Astronomy
a. A usually spherical region of interstellar matter surrounding a forming star and interacting with the star's gravitational and radiation fields.
b. The coma of a comet.
7. Mathematics A curve or surface that is tangent to every one of a family of curves or surfaces.
push the envelope
To exceed or try to exceed the existing limits of a discipline or activity.
[French enveloppe, from envelopper, to envelop, from Old French envoloper; see ENVELOP.]
Usage Note: Some people dislike the pronunciation (ŏnvə-lōp′), arguing that it is pretentious for being pseudo-French and that it is unnecessary, since there is a perfectly acceptable Anglicized pronunciation, (ĕnvə-lōp′). But the pronunciation with (ŏn) is gaining in acceptability: in our 1992 survey, 30 percent of the Usage Panel used it exclusively, and another 9 percent used it occasionally; by 2011, 46 percent selected it as their preferred pronunciation. Both forms should be considered standard.