Plain English Meaning
A straight line drawn through the airplane from nose to tail. When the airplane tips one wing down and the other up, it is rotating around this line.
Full Definition
The imaginary straight line that runs through the airplane from nose to tail, passing through the center of gravity. The airplane rotates around this axis when it rolls, which is controlled by the ailerons.
Context Anchor
Seen during before-takeoff control checks and basic flight-control discussions, especially when checking the controls that make the airplane tilt left or right.
Derivation
From Latin 'longitudo' meaning length. The longitudinal axis is the one that runs along the length of the airplane — nose to tail — which is the airplane's longest .
