Definition of: envelope

(en′və·lōp, än′-) noun
1. A case or wrapper of paper with gummed edges, for enclosing a letter or the like.
2. Any enclosing covering; a wrapper.
3. Aeron. The outer fabric covering of an aerostat.
4. Biol. The enclosing membrane of an organ.
5. Electronics The glass or metal casing of a vacuum tube.
6. Geom. A curve or surface to which another curve or surface, varying or moving according to any law, is invariably tangent.
7. Astron. A curved sheet of nebulous matter rising and expanding from the nucleus of a comet on the side toward the sun. Also en′·ve·lop. [<F enveloppe <envelopper <OF enveloper ENVELOP]