optic chiasm vs chiasma

optic chiasm and chiasma both are nouns.

Word	Noun	Adjective	Verb	Adverb
optic chiasm	Yes	No	No	No
chiasma	Yes	No	No	No

As nouns, chiasma is a hypernym of optic chiasm; that is, chiasma is a word with a broader meaning than optic chiasm:

optic chiasm: the crossing of the optic nerves from the two eyes at the base of the brain
chiasma: an intersection or crossing of two tracts in the form of the letter X

Other hypernyms of optic chiasm include chiasm, decussation.

optic chiasm (noun)	chiasma (noun)
the crossing of the optic nerves from the two eyes at the base of the brain	an intersection or crossing of two tracts in the form of the letter X