Definition of chiasma in English:
 chiasma
nounPlural chiasmata kʌɪˈazməkɪˈazməkaɪˈæzmə
1Anatomy 
The X-shaped structure formed at the point below the brain where the two optic nerves cross over each other.
 Example sentencesExamples
-  The dura that invests the optic chiasma can be traced from the chiasma along the optic nerve.
 
2Biology 
A point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.
 Example sentencesExamples
-  Genetic recombination is absent and no chiasmata are formed in bivalents.
 -  A given factor s can affect the recombination rate r through a change in the number of chiasmata occurring during meiosis.
 -  During meiosis, homologous chromosomes become physically connected by the formation of chiasmata.
 -  First, it generates crossovers between homologous chromosomes that mature into visible chiasmata.
 -  However, the observed differences may be conditional on the chromosomal location of the chiasmata.
 
Origin
  
Mid 19th century: modern Latin, from Greek chiasma 'crosspiece, cross-shaped mark', from khiazein 'mark with the letter chi'.