Grain

1. A small, dry, one-seeded fruit of a cereal grass, having the fruit and the seed walls united. Wheat, barley, rice and corn are some of the major grains.
2. The fruits of cereal grasses especially after having been harvested, considered as a group. The grain crop has been poor this year.
3. A relatively small discrete particle or crystalline mass. A single grain of sand can stop a delicate piece of machinery.
4. A small amount or the smallest amount possible. She hasn't a grain of sense in that head of hers.