Structure formed by the crossing over of the optic nerves

Optic chiasma

Explanation:

Optic chiasma is the part of the brain where the optic nerves partially cross.

For example, the optic nerve fibres on the nasal sides of each retina cross over to the opposite side of the brain at the site of optic chiasm. The same thing is with the opposite site. Since the fibres intersect at the chiasm, it has the hallmark X-shape.