cereal vs buckwheat

cereal and buckwheat both are nouns.

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

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

cereal (noun)	buckwheat (noun)
a breakfast food prepared from grain	grain ground into flour
foodstuff prepared from the starchy grains of cereal grasses	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
grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet

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