What is Grain?

Legal Definition
Grains are small, hard, dry seeds, with or without attached hulls or fruit layers, harvested for human or animal consumption. Agronomists also call the plants producing such seeds "grain crops". The two main types of commercial grain crops are cereals such as wheat and rye, and legumes such as beans and soybeans. Ubiquity of grain as a food source encouraged use of the term to describe other particles with volume or mass similar to an individual seed.

After being harvested, dry grains are more durable than other staple foods, such as starchy fruits (plantains, breadfruit, etc.) and tubers (sweet potatoes, cassava, and more). This durability has made grains well suited to industrial agriculture, since they can be mechanically harvested, transported by rail or ship, stored for long periods in silos, and milled for flour or pressed for oil. Thus, major global commodity markets exist for canola, maize, rice, soybeans, wheat, and other grains but not for tubers, vegetables, or other crops.
-- Wikipedia
Legal Definition
The general group of EXCHANGETRADED DERIVATIVES referencing wheat, corn, soybeans, maize, and sorghum. See also COMMODITY DERIVATIVE, SOFTS.
Legal Definition
Weight. The twenty-fourth part of a pennyweight.

2. For scientific purposes the grain only is used, and sets of weights are constructed in decimal progression, from 10,000 grains downward to one hundredth of a grain.
-- Bouviers Law Dictionary
Legal Definition
Corn. It signifies wheat, rye, barley, or other corn sown in the ground In Pennsylvania, a tenant for a certain term is entitled to the way-going crop. 5 inn. 289, 258; 2 Binn. 487; 2 Serg. & Rawle, 14.
-- Bouviers Law Dictionary
Legal Definition
In Troy weight, the twenty-fourth part of a pennyweight. Any kind of corn sown in the ground. See Grain rent.
-- Black's Law Dictionary
Legal Definition
Corn; cereal plants.
-- Ballentine's Law Dictionary