Envelope (noun)

1. A flat paper container, usually rectangular and foldable, used to enclose a letter or document.
2. A mathematical or geometrical curve that bounds or envelops a set of points.
3. A covering or containing structure, especially a layer of gas surrounding a celestial body.
4. A boundary or limit, especially one that restricts or encloses an area.

Origin:

From french enveloppe, meaning 'wrapper'.

Examples:

1. I put my letter in an envelope, wrote the address, and put a stamp on it.
2. The shape of the envelope of a sound wave determines the timbre of the sound.
3. The Earth's atmosphere is a thin envelope of air that surrounds the planet.
4. The economic crisis has created a new reality with tighter fiscal envelopes for governments.
5. The boundary of the envelope of an airplane's flight path is called the sonic boom.