For other uses, see Chi.

Chi (upper case Χ, lower case χ) is the 22nd letter of the Greek alphabet. Its value in Ancient Greek was an aspirated velar stop (/k^h/), but in Koine and later dialects it fricativized along with Θ and Φ. In Modern pronunciation, it is a voiceless dorsal fricative. In front of "light" vowels (e, i, oi, ai, y) it is prounounced like the German ich-laut (/ç/) or some variants of 'h' in the English 'hew' or 'human'. In front of "dark" vowels (a, o, ou) and consonants, it is pronounced like the German ach-laut (/χ/) or in Scottish 'loch'.

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.

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

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

• Part of or a subsitution for the Labarum monogram.
• The name of Jesus Christ in Christianity, as in X-mas etc.
• The last letter of the name of the TeX typesetting system.

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

• The chi-square distribution or Chi-square test in statistics.
• 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

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

Greek letters

Χ | Khi (lizherenn) | Хи | Khi | Chi | Chi | Χι | Χ | Khi | Chi | Χ | Chi (lettera) | כי | Chi (letter) | Χ | Chi | Chi (litera) | Χ | Chi | Χ