envelope

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

(noun) any wrapper or covering.

(noun) a curve that is tangent to each of a family of curves.

(noun) a natural covering (as by a fluid). Example: "The spacecraft detected an envelope of gas around the comet."

(noun) the maximum operating capability of a system (especially an aircraft). Example: "Test pilots try to push the envelope."

(noun) the bag containing the gas in a balloon. Synonyms: gasbag.