concave shape vs niche

concave shape and niche both are nouns.

Word	Noun	Adjective	Verb	Adverb
concave shape	Yes	No	No	No
niche	Yes	No	No	No

As nouns, niche is a hyponym of concave shape; that is, niche is a word with a more specific, narrower meaning than concave shape:

concave shape (noun)	niche (noun)
a shape that curves or bends inward	an enclosure that is set back or indented
	a small concavity
	a position particularly well suited to the person who or organization which occupies it
	(ecology) the status of an organism within its environment and community (affecting its survival as a species)