The optic chiasm is the part of the brain where the optic nerves partially cross, those parts of the right eye which see things on the right side being connected to the left side of 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. To compensate for lack of crossing in their brains, they cross their eyes. This is also seen in albino tigers.