principal axis,
optic axis a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted; "in a normal eye the optic axis is the direction in which objects are seen most distinctly"
Axis -
in World War II the alliance of Germany and Italy in 1936 which later included Japan and other nations; "the Axis opposed the Allies in World War II"
Axis -
A fixed one-dimensional figure, such as a line or arc, with an origin and orientation and such that its points are in one-to-onecorrespondence with a set of numbers; an axis forms part of the basis of a space or is used to position and locate data in a graph a coordinate axis.