Plain English Meaning
An imaginary vertical line running straight down through the middle of the airplane. When the airplane turns its nose to the left or right around this line, that movement is called yaw.
Full Definition
The yaw axis is the vertical axis of an aircraft, passing through the center of gravity from top to bottom. Rotation about this axis is called yaw, and it moves the nose of the aircraft left or right. Yaw is controlled primarily by the rudder.
Context Anchor
Seen in instrument flying when separating pitch, bank, and yaw movements during attitude control.