A grain is a type of simple dry fruit. A grain is moncarpelate (formed from a single carpel) and indehiscent (not opening at maturity) and resembles an achene, except that in a grain the pericarp is fused with the thin seed coat. Examples of grains are wheat, rice, and corn — the seeds of these and other grasses (Family Poaceae) are grains. The "hulls" to be separated from many grains before processing are flower bracts.
The term grain is also used in a more general sense as synonymous with cereal or cereal grains. Considering that the fruit wall and the seed are intimately fused into a single unit, and the grain is a dry fruit, it is not surprising that in general usage little concern is given to technically separating the terms "fruit" and "seed" in these fruits.