Title: Chiasm (anatomy)
Content:  As stated above, very different kinds of nerve crossings are known as chiasm. The optic chiasm of vertebrates is the best known. The optic nerve runs from the retina towards the ventral midline of the brain and crosses to the opposite side to continue as the optic tract which inserts to the optic tectum (=superior colliculus)) on the dorsal midbrain (as well as branching off to the thalamus in amniotes).
Title: Optic chiasm
Content:  The optic chiasm, or optic chiasma (Greek: χίασμα, "crossing", from the Greek χιάζω, "to mark with an X", after the Greek letter "Chi"), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes (lampreys and hagfishes), it is located within the brain. This article is about the optic chiasm of vertebrates, which is the best known nerve chiasm, but not every chiasm denotes a crossing of the body midline (e.g., in some invertebrates, see Chiasm (anatomy)). A midline crossing of nerves inside the brain is called a decussation (see Definition of types of crossings).
Title: Optic chiasm
Content:   For the different types of optic chiasm, see In all vertebrates, the optic nerves of the left and the right eye meet in the body midline, ventral to the brain. In many vertebrates the left optic nerve crosses over the right one without fusing with it. In vertebrates with a large overlap of the visual fields of the two eyes, i.e., most mammals and birds, but also amphibians, reptiles such as chameleons, the two optic nerves merge in the optic chiasm. In such a merged optic chiasm, part of the nerve fibres do not cross the midline, but continue towards the optic tract of the ipsilateral side. By this partial decussation, 
Title: Chiasm (anatomy)
Content:  the contralateral eye (i.e., the superior oblique muscle). It emerges from the dorsal aspect of the ventral midbrain, leaves the brain on the dorsal side where it crosses to the opposite side. The oculomotor nerve originates from the third nerve nucleus at the level of the superior colliculus (in non-mammalian vertebrates this is the optic tectum) in the midbrain. The rostral part of the nerve crosses the midline to merge with the part of the contralateral nerve that does not cross. Since the midline crossing occurs inside the brain, it is not strictly a chiasm but rather a decussation.
Title: Chiasm (anatomy)
Content:   In mammals and birds and other vertebrates with frontal eyes, the optic nerves do blend in the optic chiasm, and only part of the nerve fibres cross the midline. The drawings of Cajal suggest that the axons of the optic nerve may branch in the optic chiasm, and thus give off a branch both in the ipsi- and contralateral optic tract. Note, however, that such branching is not neural processing as occurs in a ganglion.
Title: Optic chiasm
Content:  In cephalopods and insects the optic tracts do not cross the body midline, so each side of the brain processes the ipsilateral eye.
Title: Chiasmal syndrome
Content:  The optic chiasm is formed by the union of the two optic nerves. The nasal fibers of each optic nerve decussate (cross) across the chiasm to the contralateral side while the temporal fibers course posteriorly to form the optic tract on the ipsilateral side. This arrangement allows the left half of the visual field to end up on the right side of the brain and the right half of the visual field to locate to the left side. The optic nerves consist of the axons from the retinal ganglion of each eye. At the chiasm, 53% of the axons from the nasal retina cross the midline to join 
Title: Visual acuity
Content:  two optic nerves come together behind the eyes at the optic chiasm, where about half of the fibers from each eye cross over to the opposite side and join fibers from the other eye representing the corresponding visual field, the combined nerve fibers from both eyes forming the optic tract. This ultimately forms the physiological basis of binocular vision. The tracts project to a relay station in the midbrain called the lateral geniculate nucleus, part of the thalamus, and then to the visual cortex along a collection of nerve fibers called the optic radiation. Any pathological process in the visual system, even in older humans 
Title: Optic nerve
Content:  the cranial space before joining the optic chiasm. There, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body. Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina; 2. orbital part (which is 
Title: Chiasm (anatomy)
Content:  In jawless vertebrates (hagfish and lamprey), the optic tracts do cross in the midline, but only after entering the ventral side of the central nervous system. After crossing the tracts insert on the dorsal optic tectum as in all other vertebrates. Therefore, given the obvious and undisputed homology, the optic chiasm is called chiasm also in these clades, even though the crossing is technically a decussation.

Question: where does the optic nerve cross the midline ​?
Answer: optic chiasm.