Plain English Meaning

It is the line that runs lengthwise down the middle of the airplane, from the nose to the tail. When the airplane tips one wing down and the other wing up, it is turning around this line.

Full Definition

The roll axis, also called the longitudinal axis, is an imaginary straight line running through the airplane from nose to tail, passing through its center of gravity. Rotation about this axis is called roll and is controlled primarily by the ailerons. When an airplane rolls, one wing moves down while the other moves up.

Context Anchor

Seen when learning the three airplane axes and how the flight controls make the airplane move.

Derivation

Longitudinal comes from the Latin longitudo meaning length. The roll axis runs along the length of the airplane, which is why it is called the longitudinal axis. Roll simply describes the motion the airplane makes around it, like a log .