Chiasma, a "crossing", from the Greek letter for X, chi (pronounced "ki", rhyming with "sky").
The term was first used by first century Romans for the X-shaped crossing of the optic nerves just anterior to the pituitary. For some reason the word fell out of favor with subsequent generations of anatomists; not until the late 1500's was it revived, reentering the anatomical lexicon for a permanent stay (although "optic chiasm" is now preferred over "optic chiasma" by about a 4 to 1 ratio; both terms are recognized by the Terminologia Anatomica.)
True to its derivation, chiasma is pronounced with a hard ch, as in "chronicle". 
Chi
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The English pronunciation of Greek letters generally makes use of the "hard" sound of each letter. Although there is some debate over this topic, the Ancient Greeks are also believed to have used the hard pronunciation whereas Modern Greek uses a "soft" pronunciation of the Greek Alphabet instead. For example, the Greek letter "X," spelled "Chi" pronounced like "Ki," as in Kite, would be pronounced like the word "He" with a semi guttural emphasis on the "H," in Modern Greek.
The word chronometer, for example, has the root word "chronos" coming from Greek meaning "time" or "year." In Modern Greek it would be spelled as "Xprovos," pronouned liked "Hronos."
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