- In an optical system, the optical axis is an imaginary line that defines the path along which light propagates through the system. For a system composed of simple lenses and mirrors, the axis passes through the center of curvature of each surface, and coincides with the axis of rotational symmetry. The optical axis is often coincident with the system's mechanical axis, but not always, as in the case of off-axis optical systems.
- In an uniaxial birefringent material, the optical axis is the axis of optical anisotropy.
- For an optical fiber, the optical axis is along the center of the fiber core, and is also known as the fiber axis.