Definition of "Envelope" :

noun: any wrapper or covering

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

noun: the bag containing the gas in a balloon

noun: the maximum operating capability of a system (especially an aircraft)

"Test pilots try to push the envelope."

noun: a natural covering (as by a fluid)

"The spacecraft detected an envelope of gas around the comet."

noun: a curve that is tangent to each of a family of curves