Cupping

Cupping is when a board curves across its width, viewed end-on, like a shallow trough. It's the most common lumber-movement defect in flatsawn boards specifically, since the growth rings pull unevenly as moisture content changes -- see Grain for why quartersawn stock resists this better. A cupped board can sometimes be flattened by planing, but only at the cost of overall thickness, since the material has to come from somewhere.