1. Of a refractive or reflective optical element, the straight line that is coincident with the axis of symmetry of the surfaces.
Note: The optical axis of a system is often coincident with its mechanical axis, but it need not be, e.g., in the case of an off-axis parabolic reflector used to transmit signals to, or receive signals from, a geosynchronous satellite. Contrast with off-axis optical system.
2. In a lens element, the straight line which passes through the centers of curvature of the lens surfaces.
3. In an optical system, the line formed by the coinciding principal axes of the series of optical elements.
4. In an optical fiber, synonym fiber axis , which is the preferred term.
Source: from Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms