Definition of Envelope

Noun: envelope 'en-vu,lowp

A flat (usually rectangular) container for a letter, thin package, etc.
Any wrapper or covering
A curve that is tangent to each of a family of curves
A natural covering (as by a fluid)
"the spacecraft detected an envelope of gas around the comet"
The maximum operating capability of a system (especially an aircraft)
"test pilots try to push the envelope"
The bag containing the gas in a balloon
- gasbag