- noun an imaginary line running through the fuselage at the centre of gravity from top to bottom, around which the aircraft rotates when it yaws
- noun a vertical reference line (Y axis) of a graph
- noun a reference line used for vertical coordinates on a graph
- An axis which is considered to be parallel to, or situated along a vertical line, object, or the like. In a Cartesian coordinate system, for instance, it is usually considered to be the y-axis.
- noun reference lines used for vertical coordinates on a graph