Plain English Meaning
An imaginary line running from the nose of the airplane straight back to the tail. When the airplane tips one wing up and the other wing down, it is rotating around this line.
Full Definition
The longitudinal axis of an airplane, running from the nose to the tail through the center of gravity. Rotation about this axis is called roll, and it is controlled by the ailerons, which raise one wing while lowering the other.
Context Anchor
You encounter this term when learning attitude control, especially when controlling wing tilt with the control wheel or stick.
Derivation
From the Old English 'rollen,' meaning to turn or revolve, and the Latin 'axis,' meaning a line about which something . The roll axis is simply the line the airplane rolls around.