Chi (letter)

==Math and science==
In [[statistics]], the term chi-squared or <math alt="χ²">\chi^2</math> has [[Chi-squared (disambiguation)|various uses]], including the [[chi-squared distribution]], the [[chi-squared test]], and [[chi-squared target models]].

In [[algebraic topology]], Chi is used to represent the [[Euler characteristic]] of a surface.

In [[neuroanatomy]], [[Chiasm (anatomy)|crossings]] of [[Peripheral nervous system|peripheral nerves]] (such as the [[optic chiasm]]) are named for the letter Chi because of its Χ-shape.<ref>{{cite book |title=The Human Brain |last=Asimov |first=Isaac |author-link= Isaac Asimov |year=1963 |publisher=Houghton Mifflin |location=Boston}}</ref>

In [[chemistry]], the [[mole fraction]]<ref>{{cite book|last=Zumdahl|first=Steven S.|title=Chemistry|url=https://archive.org/details/chemistrythediti00zumd|url-access=limited|year=2008|publisher=Cengage Learning|isbn=978-0547125329|edition=8th|page=[https://archive.org/details/chemistrythediti00zumd/page/n229 201]}}</ref><ref>{{cite book|last=Rickard|first=James N. Spencer, George M. Bodner, Lyman H.|title=Chemistry : structure and dynamics.|url=https://archive.org/details/chemistrystructu00spen_759|url-access=limited|year=2010|publisher=Wiley|location=Hoboken, N.J.|isbn=9780470587119|edition=5th|page=[https://archive.org/details/chemistrystructu00spen_759/page/n378 357]}}</ref> and [[electronegativity]]<ref>{{GoldBookRef|file=E01990|title=Electronegativity}}</ref> may be denoted by the lowercase <math alt="χ">\chi</math>.

In [[physics]], <math alt="χ">\chi</math> denotes [[electric susceptibility|electric]] or [[magnetic susceptibility]].

In [[rhetoric]], both [[chiastic structure]] (a [[literary device]]) and the [[figure of speech]] [[Chiasmus]] derive from their names from the shape of the letter Chi.

In [[mechanical engineering]], chi is used as a symbol for the [[reduction factor]] of relevant [[buckling]] loads in the [[EN 1993]], a [[European Standard]] for the design of steel structures.

In [[graph theory]], a lowercase chi is used to represent a graph's [[chromatic number]].

In [[analytic number theory]], chi is used for the [[Dirichlet character]].