gasbag vs envelope

gasbag and envelope both are nouns.

Word	Noun	Adjective	Verb	Adverb
gasbag	Yes	No	No	No
envelope	Yes	No	No	No

As nouns, gasbag and envelope are synonyms defined as:

gasbag (noun)	envelope (noun)
the bag containing the gas in a balloon	any wrapper or covering
a boring person who talks a great deal about uninteresting topics	a flat (usually rectangular) container for a letter, thin package, etc.
	the bag containing the gas in a balloon
	the maximum operating capability of a system (especially an aircraft)
	a natural covering (as by a fluid)
	a curve that is tangent to each of a family of curves