The direction of the wood fibers along a board's length. Also used loosely to refer to figure or pattern.
Plane with the grain or you'll get tearout.
Decorative patterns in wood beyond plain grain — curl, quilt, birdseye, fiddleback, burl, crotch.
Wood fibers ripped out below the surface by a blade or plane cutting against the grain direction. Often impossible to sand out — must be scraped or planed in the correct direction.