光轴是光学系统中，一条假想的线，定义（在一次近似下）光学系统如何传导光线。光线若和光轴重合，在光学系统中光将沿光轴传递。

若此光学系统有一定程度的转动对称（像相机镜头或是显微镜），光轴一般会是光学系统的旋转中心，若光学系统是由简单的透镜和反射镜组成，光轴会通过各平面的曲率中心（如焦点），和转动对称轴重合。光轴一般会和系统的机械中心重合，但也有例外，例如离轴光学系统。

若光线和光轴角度很小，而光线接近光学系统的轴，可以用几何光学中的近轴近似来处理，可以简化数学的运算。

在光纤中，光轴会和纤维芯重合，也称为光纤轴。

An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.

The optical axis is an imaginary line that defines the path along which light propagates through the system, up to first approximation. 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.