envelope

n 1: a flat rectangular paper container for papers

2: any wrapper or covering

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

4: a natural covering (as by a fluid); "the spacecraft detected

an envelope of gas around the comet"

5: the maximum operating capability of a system; "test pilots

try to push the envelope"

6: the bag containing the gas in a balloon [syn: {gasbag}]