Plain English Meaning
An imaginary vertical line running straight down through the middle of the airplane. When the airplane turns its nose left or right around this line, it is yawing.
Full Definition
The vertical axis of an airplane, passing through the aircraft from top to bottom and intersecting the longitudinal and lateral axes near the center of gravity. Rotation about the yaw axis swings the nose left or right and is controlled primarily by the rudder.
Context Anchor
Used in attitude control and basic flight control discussions when describing how the airplane moves in response to rudder input.