envelope

Noun

• the bag containing the gas in a balloon

• the maximum operating capability of a system (especially an aircraft)
  • test pilots try to push the envelope
    

• a natural covering (as by a fluid)
  • the spacecraft detected an envelope of gas around the comet
    

• a curve that is tangent to each of a family of curves

• any wrapper or covering

• a flat (usually rectangular) container for a letter, thin package, etc.