tuber vs water chestnut

tuber and water chestnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuber	Yes	No	No	No
water chestnut	Yes	No	No	No

As nouns, water chestnut is a hyponym of tuber; that is, water chestnut is a word with a more specific, narrower meaning than tuber:

tuber: a fleshy underground stem or root serving for reproductive and food storage
water chestnut: edible bulbous tuber of a Chinese marsh plant

Other hyponyms of tuber include groundnut, potato bean, wild bean, Jerusalem artichoke, yam.

tuber (noun)	water chestnut (noun)
a fleshy underground stem or root serving for reproductive and food storage	edible bulbous tuber of a Chinese marsh plant
	Chinese sedge yielding edible bulb-shaped tubers
	a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits