Grain
Grain turns up in 2 fields: paper optics, aviation. Here is what it means to the people in each one.
Grain in paper optics
The main direction in which fibers in a paper sheet are aligned, almost always the machine direction.
Because the stock jet and the wire travel the same way, more fibers line up along the machine direction, which gives the sheet its grain. Paper is stiffer and stronger along the grain and swells more across it when moisture changes. Grain also matters for optical work, because gloss, smoothness and readings taken with directional geometry often differ along and across it.
"Cut it grain parallel to the spine or the bindery will send the whole job back."
Grain in aviation
The solid mass of propellant cast inside a solid rocket motor.
A grain is the shaped block of solid propellant housed inside a rocket motor casing. It contains both fuel and oxidizer mixed together and cast into a specific geometry, such as a star or cylinder with a hollow core. The cross-sectional shape of the grain dictates the surface area exposed to combustion, which precisely controls the thrust profile as the rocket burns.
"The star-shaped grain gives us high initial thrust right off the pad before tapering down during max Q."