The word grain has a great many meanings, most being descriptive of a small piece or particle. For examples:
In geology, a grain is a single particle of rock or other material, such as sand or salt; see grain size.
In agriculture, a grain is a type of simple dry fruit technically called a caryopsis. Such crops are often called cereals (grain can also refer to other types of seeds, though this is technically less correct).
In golf, the direction in which individual blades of grass lie, usually in reference to a putting green.
In rocket science, grain refers to the solid propellant of a rocket (either solid fuelled or hybrid), more or less a hollow cylinder, sometimes textured inside, and possibly very large.
