envelope vs curved shape

envelope and curved shape both are nouns.

Word	Noun	Adjective	Verb	Adverb
envelope	Yes	No	No	No
curved shape	Yes	No	No	No

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

Other hypernyms of envelope include curve.

envelope (noun)	curved shape (noun)
any wrapper or covering	the trace of a point whose direction of motion changes
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