Real Dictionary

grain
countable noun: A grain of something such as sand or salt is a tiny hard piece of it.
countable noun: A grain of wheat, rice, or other cereal crop is a seed from it.
variable noun: Grain is a cereal crop, especially wheat or corn, that has been harvested and is used for food or in trade.

grain in American English
a small, hard seed or seedlike fruit, esp. that of any cereal plant, as wheat, rice, corn, rye, etc.
noun: a small, hard seed, esp. the seed of a food plant such as wheat, corn, rye, oats, rice, or millet

grain in British English
noun: the small hard seedlike fruit of a grass, esp a cereal plant