noun, plural chiasmas, chiasmata [kahy-az-muh-tuh] .
Anatomy. a crossing or decussation, as that of the optic nerves at the base of the brain.
Cell Biology. a point of overlap of paired chromatids at which fusion and exchange of genetic material take place during prophase of meiosis.
Also, chiasm [kahy-az-uhm] .

1830–40; < Greek: crosspiece of wood, cross-bandage, equivalent to chi chi1 + -asma noun suffix

chiasmal, chiasmic, chiasmatic [kahy-az-mat-ik] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
chiasma or chiasm (kaɪˈæzmə, ˈkaɪæzəm)
n , pl -mas, -mata, -asms
1.  cytology the cross-shaped connection produced by the crossing over of pairing chromosomes during meiosis
2.  anatomy the crossing over of two parts or structures, such as the fibres of the optic nerves in the brain
[C19: from Greek khiasma wooden crosspiece, from khiazein to mark with an X, from khichi1]
chiasm or chiasm (kaɪˈæzmə, ˈkaɪæzəm, -mətə)
[C19: from Greek khiasma wooden crosspiece, from khiazein to mark with an X, from khichi1]
chi'asmal or chiasm
chi'asmic or chiasm

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

chiasma chi·as·ma (kī-āz'mə) or chi·asm (kī'āz'əm)
n. pl. or chi·as·mas, chi·as·ma·ta (-mə-tə), chi·as·ms

  1. A crossing or intersection of two tracts, as of nerves or ligaments.

  2. The point of contact between paired chromatids during meiosis, resulting in a cross-shaped configuration and representing the cytological manifestation of crossing over.

chi·as'mal or chi·as'mic or chi'as·mat'ic (-māt'ĭk) adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Within the chiasma, the optic nerves undergo a partial decussation.
The fibers forming the medial part of each tract and posterior part of the chiasma have no connection with the optic nerves.
The optic chiasma is formed by the meeting and partial decussation of the fibers of the two optic nerves.
Behind the chiasma the fibers grow backward as the optic tracts to the thalami and mid-brain.
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