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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. [JP1] 3. In an optical system, the line formed by the coinciding principal axes of the series of optical elements. [JP1] 4. In an optical fiber, synonym fiber axis , which is the preferred term.