Technically, a grain is an edible dried seed, from a plant of the grass family (Poaceae, Gramineae).
Plants of the grass family that yield edible dried seeds are collectively known as cereals and the edible seed is called a cereal grain.
Common cereals include wheat, rye, oat, barley, rice, millet, finger millet, sorghum or corn (maize). They are all grasses, classified under monocotyledons.
Buckwheat is not a grain because the species that produces it (Fagopyrum esculentum), does not belong to the grass family. it belongs to the family Polygonaceae together with Rhubarb (Rheum rhabarbarum)
Buckwheat is not a cereal, even though its seeds are edible in dried form.
It’s usually referred to as a Pseudo-cereal (meaning false cereal) or a Pseudo-grain (meaning a false grain).
Buckwheat is not related to wheat or any of the plants in the grass family.
Botanically it’s known as Fagopyrum esculentum Moench.