What does chiasma mean?
A chiasma is a crossing point — literally an X-shaped intersection, named after the Greek letter chi. The word appears most often in biology. In genetics, chiasmata are the visible points where paired homologous chromosomes touch and exchange segments during meiosis, a process essential to genetic recombination and variation. In anatomy and optics, the best-known instance is the optic chiasma, the structure at the base of the brain where the optic nerves partially cross so that each hemisphere receives information from both eyes. More generally, the term can describe any crossing of lines or structures resembling an X. Because of its technical character, chiasma belongs to scientific and academic registers rather than casual conversation, though its vivid image makes it easy to grasp once explained.
nounA crossing or intersection of two lines, parts, or structures in the shape of the Greek letter chi (X); in genetics, the point at which paired homologous chromosomes exchange segments during meiosis.
- (genetics) The visible point of contact where two homologous chromosomes exchange genetic material during meiosis.
- (anatomy/optics) A crossing of nerve fibres or other structures in an X-shape, as in the optic chiasma.
- (general) Any crossing or intersection of two lines or things resembling the letter X.
"Under the microscope, each chiasma marked the spot where the homologous chromosomes had swapped genetic material."
"Each chiasma along the tetrad represents a site of genetic recombination."
"Injuries near the optic chiasma can produce distinctive losses in the visual field."
The standard plural follows the Greek pattern, like 'stigma/stigmata'; 'chiasmas' is rare and generally avoided in scientific writing.
"Several chiasmata were visible along the bivalent during metaphase I."
The X shape of a chiasma is literally named after the Greek letter chi — the same X that gave us 'Xmas' and 'X-rays'.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chiasma
Chiasma comes from the Greek 'khiasma', meaning a cross-piece or arrangement in the shape of the letter chi (Χ), itself derived from 'khiazein', to mark with a chi. It entered English in the early 19th century through scientific Latin. It shares its root with the rhetorical term 'chiasmus' and the adjective 'chiastic', both describing reversed or crossed arrangements of words.
Related word forms
How chiasma is actually used
A technical term used mainly in genetics, cytology, anatomy, and optics rather than everyday speech. In British English it is often pronounced with an initial /kɪ/ sound, while American English typically favours /kaɪ/. In genetics contexts the plural 'chiasmata' is strongly preferred.
Easily confused with chiasma
Chiasmus is a rhetorical device in which words or ideas are repeated in reversed order, while a chiasma is a physical point of crossing, such as between chromosomes.
A chasm is a deep fissure or gap in the earth or a figurative divide, whereas a chiasma is a crossing point shaped like an X.