What is the meaning of envelope?

noun

• 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. )
  • usage: "the spacecraft detected an envelope of gas around the comet"
• The maximum operating capability of a system (especially an aircraft. )
  • usage: "test pilots try to push the envelope"
• The bag containing the gas in a balloon.
  • synonyms: gasbag