Grain is a popular name of a botanical term “caryopsis” and an agricultural term used for crops harvested for human or livestock consumption that may include legumes such as beans.
Grain (as in caryopsis) is a one-seeded dry fruit (also commonly known as “kernel”) that doesn’t open when mature (indehiscent). The fruit component is dry, looks more like a woody layer (fruit wall) and is stuck together with the seed coat (the hard outer layer of a seed).
All members of the Grass family (Poaceae family) are typical caryopsis (or grains in a botanical sense) and include barley, oats, rice, rye, corn, sorghum, millet, and wheat (and its relative spelt). (1)
Example of a caryopsis: a sweet corn round grain is a kernel (fruit) and the ear of the corn is just a collection of those fruits (kernels).
In the day-to-day communication, we think of grains as “cereal grains” – those dry fruit which share similar culinary use and nutritional characteristics and include either the Grass family Poaceae or pseudo-grasses (such as quinoa, amaranth and buckwheat). Legumes such as beans, however, may be referred to as grains in some agricultural contexts since it is usually considered under Legume food group. Grains on this website refer to the culinary “cereal grains”.