In anatomy a chiasm is the spot where two structures cross, forming an X-shape (from Greek letter χ, Chi). Examples of chiasms are:

Different types of crossings of nerves are referred to as chiasm:

Note that in the third type there is no crossing of the mid sagittal plane. Only in the first type, the crossing is complete.

There are other kinds of crossings of nerve fibres. The chiasm is distinguished from a decussation, which is a crossing of nerve fibres inside the central nervous system. A chiasm also differs from a ganglion in that axons run through it without making any synapses. A chiasm is thus not a nervous processing centre.

By far the most widely known chiasm is the optic chiasm in vertebrate animals, including humans.

Chiasms are found in vertebrates but also in invertebrates. The optic chiasm in vertebrates can be of type I or II. However, an optic chiasm of type III is found in many insects and in cephalopods.

In vertebrates, three of the cranial nerves show a chiasm.

The optic chiasm of vertebrates involves the optic tract. The trochlear nerve is a motor nerve that innervates one of the muscles that move 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.