chiasma opticum vs decussation

chiasma opticum and decussation both are nouns.

Word	Noun	Adjective	Verb	Adverb
chiasma opticum	Yes	No	No	No
decussation	Yes	No	No	No

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

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

Other hypernyms of chiasma opticum include chiasm, chiasma.

chiasma opticum (noun)	decussation (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