Step 1: Where it lies.
Every thin lens has one special point on its principal axis, its optical center \(O\), which for a symmetric lens coincides with the geometric center.
Step 2: The defining behaviour.
Consider a ray aimed at \(O\). At the point where it meets the two lens surfaces, those surfaces are effectively parallel (like the two faces of a thin glass slab). A ray crossing a parallel-sided slab emerges parallel to its original path; for a thin lens the slab thickness is negligible, so the ray comes out along the same straight line, that is, undeviated.
Step 3: Why it is useful.
This undeviated ray is one of the three principal rays used to locate images in geometrical optics, which makes the optical center an important reference point.
\[\boxed{\text{Central axial point giving zero deviation to a passing ray}}\]