noun

1. a thing that envelops; wrapper; covering
2. a folded paper container as for a letter, usually with a gummed flap for sealing
3. 1. the outer covering of a rigid airship
   2. the bag that contains the gas in a balloon or nonrigid airship
4. the set of limitations, as for a particular aircraft, system, etc., within the boundaries of which it can operate safely and efficiently
5. a set of limits or boundaries: usually in the informal phrase push the envelope, to go beyond or attempt to go beyond established rules, limits, expectations, etc.
6. Astron. a cloudy mass surrounding the nucleus of a comet; coma
7. Biol. any enclosing membrane, skin, shell, etc.
8. Math. a curve that is tangent to every one of a family of curves, or a surface that is tangent to every one of a family of surfaces

Etymology: Fr & OFr enveloppe < OFr envoluper: see envelop