envelope envelope /ˈɛnvəˌloʊp/

noun

1. () A flat (usually rectangular) container for a letter, thin package, etc.  ( envelope )
   a flat (usually rectangular) container for a letter, thin package, etc.
    I sealed the letter in the envelope and dropped it in the mailbox
   I sealed the letter in the envelope and dropped it in the mailbox
   
   
2. () Any wrapper or covering  ( envelope )
   any wrapper or covering
    She sealed the letter and slid it into an envelope to mail.
   She sealed the letter and slid it into an envelope to mail.
   
   
3. The bag containing the gas in a balloon  ( envelope , gasbag )
   the bag containing the gas in a balloon
   
4. The maximum operating capability of a system (especially an aircraft)  ( envelope )
   the maximum operating capability of a system (especially an aircraft)
    Test pilots try to push the envelope
   test pilots try to push the envelope
   
5. A natural covering (as by a fluid)  ( envelope )
   a natural covering (as by a fluid)
    The spacecraft detected an envelope of gas around the comet
   the spacecraft detected an envelope of gas around the comet
   
6. A curve that is tangent to each of a family of curves  ( envelope )
   a curve that is tangent to each of a family of curves