Final answer:
The line along an aircraft's center line is known as the longitudinal axis, which is important for the aircraft's balance, orientation, and response to control inputs.
Step-by-step explanation:
The line along an aircraft's longitudinal axis or center line is designated the longitudinal axis. This axis runs from the nose to the tail of the aircraft and is central to the aircraft's balance and orientation. The longitudinal axis is crucial in aerodynamics and aircraft design since it defines the symmetry of the airplane and influences how the aircraft responds to control inputs, particularly those which affect pitch, such as elevators or a canard. The line along the aircraft's longitudinal axis or center line is called the longitudinal axis.