Warp
Any deviation from a flat, true plane in a piece of lumber, including bow, crook, cup, and twist, caused by uneven drying, internal stresses, or irregular grain.
In-Depth Explanation
Warp is a broad term encompassing several distortion types: bow is a curve along the face lengthwise, crook is a curve along the edge lengthwise, cup is a curve across the width, and twist is a spiral distortion. These defects result from differential shrinkage as wood dries because wood shrinks more tangentially than radially. Proper stickering, controlled kiln drying, and acclimating lumber to its installation environment all help minimize warp. When selecting lumber, sight down the length and across the width of each board to check for warp. Mild warp can sometimes be corrected through planing or strategic cross-cutting.
Usage Example
After sitting in a humid garage for two weeks, several pine boards developed noticeable cup warp and needed to be planed flat.