ENVELOPE

Definition

1. noun a flat (usually rectangular) container for a letter, thin package, etc..
2. noun any wrapper or covering.
3. noun a curve that is tangent to each of a family of curves.
4. noun a natural covering (as by a fluid).
5. noun the maximum operating capability of a system (especially an aircraft).
6. noun the bag containing the gas in a balloon. — synonyms: gasbag