buckwheat vs cereal

buckwheat and cereal both are nouns.

Word	Noun	Adjective	Verb	Adverb
buckwheat	Yes	No	No	No
cereal	Yes	Yes	No	No

As nouns, cereal is a hypernym of buckwheat; that is, cereal is a word with a broader meaning than buckwheat:

Other hypernyms of buckwheat include food grain, grain.

buckwheat (noun)	cereal (noun)
grain ground into flour	a breakfast food prepared from grain
a member of the genus Fagopyrum; annual Asian plant with clusters of small pinkish white flowers and small edible triangular seeds which are used whole or ground into flour	foodstuff prepared from the starchy grains of cereal grasses
	grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet

buckwheat (adjective)	cereal (adjective)
	made of grain or relating to grain or the plants that produce it