the straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis. The path of a light ray along this axis is perpendicular to the surfaces and, as such, will be unchanged. All other ray paths passing through a lens and its optical centre (the geometrical centre of a thin lens) are called secondary axes. The optical axis of a curved mirror passes through its geometric centre and its centre of curvature.

The term optical axis should not be confused with optic axis, a term used in crystallography.