Plain English Meaning
An imaginary up-and-down line through the middle of the airplane. When the airplane pivots around this line, the nose swings left or right, like a weather vane turning.
Full Definition
The vertical axis of an airplane, passing through the center of gravity from top to bottom, around which the airplane rotates left or right (nose swinging side to side). Movement around this axis is called yaw and is controlled by the rudder.
Context Anchor
Seen in flight-control discussions when learning how rudder pedal movement turns the airplane’s nose left or right.