"envelope" in the noun sense
1. envelope
a flat (usually rectangular) container for a letter, thin package, etc.
2. envelope
any wrapper or covering
3. envelope
a curve that is tangent to each of a family of curves
4. envelope
a natural covering (as by a fluid
"the spacecraft detected an envelope of gas around the comet"
5. envelope
the maximum operating capability of a system (especially an aircraft
"test pilots try to push the envelope"
6. envelope, gasbag
the bag containing the gas in a balloon
Source: WordNet® (An amazing lexical database of English)
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