Definitions for "Longitudinal axis"
A line fore and aft through the centre of gravity of an aeroplane and parallel to the line of flight.
The fore-and-aft line through the center of a space vehicle.
The line running fore and aft through the centre of gravity of an aeroplane parallel to the line of flight.
An imaginary line running through an aircraft from nose to tail.
The axis extending through the center of the fuselage from the nose to the tail. The longitudinal axis is sometimes called the "x" axis. Roll is a motion around the longitudinal axis.
An imaginary reference line running down the center of the aircraft between the nose and tail.