Plain English Meaning
A make-believe line drawn down the exact middle of the airplane's body, from the front of the nose to the back of the tail. It's used as a reference point to describe where things are located on the airplane.
Full Definition
An imaginary straight line running lengthwise through the middle of the aircraft's fuselage, from nose to tail, used as a reference axis for measuring balance, alignment, and the location of components on the airplane.
Context Anchor
Seen in weight and balance, stability, and center-of-gravity discussions when describing where weight is located across the width of the aircraft.