tuber vs groundnut

tuber and groundnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuber	Yes	No	No	No
groundnut	Yes	No	No	No

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

tuber: a fleshy underground stem or root serving for reproductive and food storage
groundnut: nutlike tuber; important food of Native Americans

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

tuber (noun)	groundnut (noun)
a fleshy underground stem or root serving for reproductive and food storage	nutlike tuber; important food of Native Americans
	pod of the peanut vine containing usually 2 nuts or seeds; `groundnut' and `monkey nut' are British terms
	a North American vine with fragrant blossoms and edible tubers; important food crop of Native Americans