Plural of chiasm; the crossing or intersection of two structures, especially the optic chiasm where nerves cross in the brain and eyes.
From Greek 'chiasmos' (crossing), from 'chi' (the Greek letter X), because the crossed structures resembled the X-shaped letter.
The optic chiasm is where the nerves from your two eyes cross over in your brain—if you didn't have this X-shaped crossover, you'd see two separate pictures instead of one combined view!
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