principal axis(n = noun.cognition) 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"
is a kind of axis

axis, axis of rotation, axis vertebra, coordinate axis, major axis, minor axis, optic axis, principal, principal diagonal, principal investigator, principal sum, school principal, semimajor axis, semiminor axis