Final answer:
Longitudinal stability is the stability about the axis parallel to the line of flight in objects like aircraft or rockets. It ensures a stable position and prevents uncontrollable pitching or rotation.
Step-by-step explanation:
Stability about the axis which runs parallel to the line of flight is referred to as longitudinal stability. This type of stability refers to the ability of an object, like an aircraft or a rocket, to maintain a stable position or attitude along the axis of its motion. It ensures that the object remains balanced and does not pitch or rotate uncontrollably.