The word grain has a great many meanings, most being descriptive of a small piece or particle. For examples:

• a grain of sand or salt; see grain size for granule
• a grain is a type of simple dry fruit technically called a caryopsis. In agriculture, such crops are often called cereals (grain can also refer to other types of seeds, though this is technically less correct).
• in materials science (especially metallurgy), a grain is a single crystal inside solid-state matter, also referred to as crystallite
• a very small unit of mass called a grain
• the fineness of resolution for photographic film (i.e., graininess of a picture)
• wood-grain, and as a description of personality or style from wood-working (i.e. going against the grain)
• also granule, from the diminutive of Latin granum, meaning "grain".
• Granules are short-lived concection cell structures on the visible surface of the Sun.
• There is also an Isle of Grain in Kent, England, on which lies the village of Grain.
• One of the stocks in the game Stock Ticker
• in golf, the direction in which individual blades of grass, usually in reference to a putting green

However, it also means the piece of propellant of a solid rocket, more or less a hollow cylinder, sometimes very large.