envelope vs natural covering

envelope and natural covering both are nouns.

Word	Noun	Adjective	Verb	Adverb
envelope	Yes	No	No	No
natural covering	Yes	No	No	No

As nouns, natural covering is a hypernym of envelope; that is, natural covering is a word with a broader meaning than envelope:

Other hypernyms of envelope include cover, covering.

envelope (noun)	natural covering (noun)
any wrapper or covering	a natural object that covers or envelops
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