Read the dictionary definition of envelope. All definitions for this word.
1. any wrapper or covering
2. a flat (usually rectangular) container for a letter, thin package, etc.
3. the bag containing the gas in a balloon
4. the maximum operating capability of a system (especially an aircraft)
1. test pilots try to push the envelope
5. a natural covering (as by a fluid)
1. the spacecraft detected an envelope of gas around the comet
6. a curve that is tangent to each of a family of curves