envelope

• n.
  • a flat rectangular paper container for papers
  • 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

    "test pilots try to push the envelope"

  • the bag containing the gas in a balloon

    同义词： gasbag

以上来源于: WordNet