Noun

1. The rotation of an aircraft, ship, or missile about its vertical axis so as to cause the longitudinal axis of the aircraft, ship, or missile to deviate from the flight line or heading in its horizontal plane.
2. The angle between the longitudinal axis of a projectile at any moment and the tangent to the trajectory in the corresponding point of flight of the projectile.
3. An act of yawing.
4. A vessel's motion rotating about the vertical axis, so the bow yaws from side to side; a characteristic of unsteadiness.
5. The extent of yawing, the rotation angle about the vertical axis

   the yaw of an aircraft

Verb

1. To turn about the vertical axis while maintaining course.
2. To swerve off course to port or starboard.
3. To steer badly, zigzagging back and forth across the intended course of a boat; to go out of the line of course.