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Look up Χ, χ in Wiktionary, the free dictionary.

Greek alphabet
Αα Alpha	Νν Nu
Ββ Beta	Ξξ Xi
Γγ Gamma	Οο Omicron
Δδ Delta	Ππ Pi
Εε Epsilon	Ρρ Rho
Ζζ Zeta	Σσ Sigma
Ηη Eta	Ττ Tau
Θθ Theta	Υυ Upsilon
Ιι Iota	Φφ Phi
Κκ Kappa	Χχ Chi
Λλ Lambda	Ψψ Psi
Μμ Mu	Ωω Omega
Obsolete letters
Ϝϝ Digamma	Ϙϙ Qoppa
Ϛϛ Stigma	Ϡϡ Sampi
Ϻϻ San	Ϸϸ Sho
Greek diacritics

For other uses, see Chi.

Chi (uppercase Χ, lowercase χ) is the 22nd letter of the Greek alphabet. Its value in Ancient Greek was an aspirated velar stop /kʰ/ (in the Western Greek alphabet: /ks/).

In Koine Greek and later dialects it became a fricative along with Θ and Φ. In Modern Greek, is has two distinct pronunciations: In front of high or front vowels (e, i, oi, ai, y) it is pronounced as a voiceless palatal fricative /ç/, as in German ich or like the 'h' in some pronunciations of the English words 'hew' and 'human'. In front of low or back vowels (a, o, ou) and consonants, it is pronounced as a voiceless uvular fricative /χ/.

In the system of Greek numerals it has a value of 600.

In ancient times, some dialects of Greek used the chi instead of xi to represent the /ks/ sound. This was borrowed into the early Latin language, which led to the letter X being used for the same sound in Latin, and the modern languages which use the Latin alphabet.

Chi also was included into Cyrillic alphabet (as letter Х), with the phonetic value /x/ or /h/.

The upper-case letter Χ is used as the symbol for:

• Part of or a substitution for the Labarum monogram.
• The shorthand version of Jesus Christ in Christianity, as in X-mas etc. Written oaths and other contracts were once made valid by writing on them an Χ for Christ and then kissing it, so the letter chi (and Roman ex) came to stand for both a kiss and a signature.
• The last letter of the name of the TeX typesetting system.

The lower-case letter χ is used as the symbol for:

• In probability theory, chi distribution a continuous probability distribution.
• In hypothesis testing the chi-square distribution or Chi-square test, a commonly used test statistic.
• The electric susceptibility (sometimes with a subscript e) and magnetic susceptibility in physics.
• The chromatic number and the chromatic index (when shown with a prime mark or a subscript 1) of a graph in graph theory.
• The Euler characteristic in algebraic topology.
• The voiceless uvular fricative (IPA) in phonology.
• An unsigned gathering in formal bibliographic collation.
• The mole fraction of a solution in chemistry.
• The characteristic function of a subset in mathematics
• The characteristic polynomial of a square matrix.

Chi is the basis for the name Chiastic structure and the name of Chiasmus.

In Plato's Timaeus, it is explained that the two bands which form the soul of the world cross each other like the letter Χ.

The chiasma, an x-shaped connection of the optic nerves leading to the eye, got its name from the letter chi because of its shape. Source; Isaac Asimov, The Human Brain, 1963.