The optic chiasm (Greek χίασμα, "crossing", from the Greek χλαζειν 'to mark with an X', after the Greek letter 'Χ', chi) is the part of the brain where the optic nerves partially cross. Specifically, the nerves connected to the right eye that attend to the right visual field cross with the nerves from the left eye that attend to the left visual field. The parts of both eyes that attend to the right visual field are processed in the left visual system in the brain, and vice versa.

In Siamese cats with certain genotypes of the albino gene, this wiring is disrupted, with less of the nerve-crossing than is normal, as a number of scholars have reported. (OMIA) To compensate for lack of crossing in their brains, they cross their eyes (strabismus). This is also seen in albino tigers, as Guillery & Kaas report.