1. A flat (usually rectangular) container for a letter, thin package, etc.
   "He sealed the letter in an envelope before mailing it"
    
2. Any wrapper or covering
   "The seed was enclosed in a protective envelope"
    
3. The bag containing the gas in a balloon
   "The hot air balloon's envelope expanded as it was filled";
   - gasbag
    
4. A natural covering (as by a fluid)
   "the spacecraft detected an envelope of gas around the comet"
    
5. A curve that is tangent to each of a family of curves
   "The mathematician plotted the envelope of the family of curves"
    
6. The maximum operating capability of a system (especially an aircraft)
   "test pilots try to push the envelope"

Derived forms: envelopes

Type of: bag, container, cover, covering, curve, curved shape, natural covering, operating capability, performance capability, wrap, wrapper, wrapping

Part of: balloon

Encyclopedia: Envelope