From ZooTerms (Dictionary of Invertebrate Zoology)
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chiasma (noun; plural chiasmata; Greek chiasma, cross): 1. (Arthropoda: Insecta) A crossing of nerves in the nerve center. a. External (outer): a crossover between the lamina ganglionaris and the medulla externa. b. Internal (inner): a crossover between medula externa and medula internal.

2. An X-shaped chromosome formation seen in meiotic cell division, due to breakage, exchange, and reciprocal fusion of equivalent segments of homologous chromatids.