χ | chi

The letter χ (chi) was a Greek addition to the alphabet, placed after the letters of Phoenician origin, along with φ, ψ and ω. According to Wikipedia,

In Ancient Greek, ‘Χ’ and ‘Ψ’ were among several variants of the same letter, used originally for /kʰ/ and later, in western areas such as Arcadia, as a simplification of the digraph ‘ΧΣ’ for /ks/. In the end, more conservative eastern forms became the standard of Classical Greek, and thus ‘Χ’ (Chi) stands for /kʰ/ (later /x/). However, the Etruscans had taken over ‘Χ’ from western Greek, and it therefore stands for /ks/ in Etruscan and Latin.

The Greek χ gave rise to the Latin X, Gothic enguz 𐍇 and Cyrillic Х.

The lower-case χ has a number of uses in maths and sciences:

• in biology: Chi site or χ site, a DNA sequence that serves as a recombination hot spot (“Chi” is an abbreviation of “crossover hotspot instigator”)
• in chemistry: electronegativity
• in graph theory: chromatic number χ(G) for a graph G
• in mathematics: the Euler–Poincaré characteristic of a surface
• in statistics: χ² or chi-square
• in thermodynamics: a free energy parameter known as the χ-parameter, employed in the Flory–Huggins solution theory

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