Yaw is the side-to-side rotation of a vehicle around its vertical axis. Think of spinning in an office chair: that's yaw. When a plane's nose swings left or right while the wings stay level, or when a boat's bow turns away from its course, that's yaw in action.

Pilots, sailors, and engineers care about yaw because vehicles need to move in straight lines or controlled turns. A plane uses its rudder (a vertical flap on the tail) to control yaw, and a boat uses its rudder too. Without controlling yaw, a vehicle would constantly drift off course.

Yaw is one of three types of rotation that matter for moving vehicles. The other two are pitch (nose up or down) and roll (tilting side to side). Together, these three rotations describe every possible way something can turn in three-dimensional space. If you've ever felt a plane or boat suddenly swing sideways in wind or waves, you've experienced unwanted yaw. Good pilots and captains work to minimize it, keeping their passengers comfortable and their vehicles on course.