envelope

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

verb

To surround or enclose.