Grain

The word grain has several meanings, most being descriptive of a small piece or particle.

Particle-related

The primary definition is the agricultural one, with the others derived from it.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• In agriculture, a grain is the seed of a grass, a simple dry fruit technically called a caryopsis. Such crops are often called cereals. Grain can also refer to other types of small seeds, though this is technically less correct.
• In units of measurement, the grain is a unit of mass, originally based on the weight of one grain of barley. One gram of metric weight equals 15.4323584 grains. It was used in the past for older drugs such as aspirin, for bullets, and for precious metals.
• In geology, a grain is a single particle of rock or other material, such as sand or salt; see grain size.
• In materials science (especially metallurgy), a grain is a single crystal inside solid-state matter, also referred to as crystallite — related to the obsolete term "corn" as in corned beef.
• In photography, the fineness of image resolution for photographic film (i.e., graininess of a picture), originally related to the (crystallographic) grain size of silver iodide.