Noun
a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted
Source: WordNetin a normal eye the optic axis is the direction in which objects are seen most distinctly Source: Internet
If an object is asymmetric about its principal axis of rotation, the moment of inertia with respect to each coordinate direction will change with time, while preserving angular momentum. Source: Internet
For triclinic, orthorhombic, and cubic crystal systems the axis designation is arbitrary and there is no principal axis. Source: Internet
In monoclinic, rhombohedral, tetragonal, and trigonal/hexagonal systems there is one unique axis (sometimes called the principal axis) which has higher rotational symmetry than the other two axes. Source: Internet
The basal plane is the plane perpendicular to the principal axis in these crystal systems. Source: Internet
The concept was introduced by Leonhard Euler in his 1765 book Theoria motus corporum solidorum seu rigidorum; he discussed the moment of inertia and many related concepts, such as the principal axis of inertia. Source: Internet