Chiasm \Chi"asm\, Chiasma \Chi*as"ma\, n. [NL. chiasma, fr. Gr. ? two lines placed crosswise, fr. ? to mark with a [chi].]

(Anat.) A commissure; especially, the optic commissure, or crucial union of the optic nerves. -- Chi*as"mal, a..

Source: Webster's Revised Unabridged Dictionary (1913)