envelope in WordNet English dictionary

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