Bowing

Bowing is when a board curves along its length, viewed from the edge -- flat across the width but arched like a longbow from end to end. It's one of the four common lumber-movement defects alongside cupping, twist, and crook, usually caused by uneven drying or grain that wasn't stable to begin with. A moderate bow can often be planed or jointed out; a severe one may mean too much material gets removed to keep the board usable at its intended thickness.