Plain English Meaning
Keeping the nose of the airplane pointed where you want it, mainly by using the rudder pedals to stop the airplane from swinging left or right.
Full Definition
The pilot's ability to control the airplane's heading and yaw axis using the rudder, keeping the nose pointed where intended and preventing unwanted yaw or rotation about the vertical axis.
Context Anchor
Encountered during low-speed flight, stall practice, spin awareness training, takeoff, landing, and any where the airplane must aligned with the intended path.