Definitions of envelope:

• noun:   any wrapper or covering
• noun:   a flat rectangular paper container for papers
• noun:   the bag containing the gas in a balloon
• noun:   the maximum operating capability of a system

  Example: "Test pilots try to push the envelope"

• noun:   a natural covering (as by a fluid)

  Example: "The spacecraft detected an envelope of gas around the comet"

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