Plain English Meaning
A line running straight down through the middle of the aircraft. When the aircraft turns its nose left or right around this line, that movement is called yaw.
Full Definition
An imaginary straight line passing vertically through the aircraft's center of gravity, from top to bottom, around which the aircraft rotates in yaw. Movement around this axis swings the nose left or right and is controlled primarily by the rudder.
Context Anchor
Seen in attitude instrument flying when learning how an aircraft moves around its three axes and how yaw is shown or corrected during flight.
Derivation
Vertical comes from the Latin verticalis, meaning 'overhead' or 'at the top .' Yaw is an old English nautical word meaning to deviate from a . Together they a vertical line about which the aircraft swings .