chiasma noun. [kʌɪ'azmǝ] Pl. -mas, -mata [-mǝtǝ]. M19.
[mod. Latin from Greek khiasma crosspiece, decussation, from khiazein: see CHIASMUS.]
1. Anatomy. An intercrossing or decussation; esp. (in full optic chiasma) the commissure formed between the two optic nerves. M19.

2. Cytology. A structure formed by chromosomes during meiosis, at which crossing over occurs. E20.

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