In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin. See for examples Decussation of pyramids and sensory decussation. Decussation describes the point where the nerves cross from one side of the brain to the other, and typically the nerves from the left side of the body decussate to the right side of the brain and the nerves from the right side of the body decussate to the left brain, however depending on the function of the nerves the level of decussation is variable. In neuroanatomy the term chiasma is reserved for the crossing of nerves outside the brain, such as the optic chiasm.
In botanicalleaf taxology, the word decussate describes an opposite pattern of leaves which has successive pairs at right angles to each other (i.e. rotated 90 degrees along the stem when viewed from above). In effect, successive pairs of leaves cross each other. Basil is a classic example of a decussate leaf pattern.
In taxonomic description where decussate markings or structures occur, names such as decussatus or decussata or otherwise in part containing "decuss..." are common, especially in the specific epithet.
The origin of the contralateral organization, the optic chiasm and the major decussations on the nervous system of vertebrates has been a long standing puzzle to scientists. For long the visual map theory of Ramón y Cajal has been the most popular theory, see also  for an english summary. More recently, scientists have realized that this theory has some severe flaws. According to the current theory, the decussations are caused by an axial twist which makes it so that the anterior head, along with the forebrain, is turned by 180° with respect to the rest of the body.
^Ramón y Cajal, Santiago (1898). "Estructura del quiasma óptico y teoría general de los entrecruzamientos de las vías nerviosas. (Structure of the Chiasma opticum and general theory of the crossing of nerve tracks)" [Die Structur des Chiasma opticum nebst einer allgemeine Theorie der Kreuzung der Nervenbahnen (German, 1899, Verlag Joh. A. Barth)]. Rev. Trim. Micrográfica (in Spanish). 3: 15–65.
^Llinás, R.R. (2003). "The contribution of Santiago Ramón y Cajal to functional neuroscience". Nat. Rev. Neurosci. 4 (1): 77–80. doi:10.1038/nrn1011. PMID12511864.
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