noun (n)

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

  source: wordnet30
• any wrapper or covering(noun.artifact)

  source: wordnet30
• a curve that is tangent to each of a family of curves(noun.shape)

  source: wordnet30
• a natural covering (as by a fluid)(noun.object)

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

  source: wordnet30
• the maximum operating capability of a system (especially an aircraft)(noun.attribute)

  Example:
  Test pilots try to push the envelope.
  

  source: wordnet30
• the bag containing the gas in a balloon(noun.artifact)

  Synonym:
  gasbag
  

  source: wordnet30
• That which envelops, wraps up, encases, or surrounds; a wrapper; an inclosing cover; esp., the cover or wrapper of a document, as of a letter.(noun)

  source: webster1913