Chi (letter)

The chi (Greek neuter Χῖ, majuscule Χ, minuscule χ, usual pronunciation when naming the letter: [çi:]) is the 22nd letter of the Greek alphabet and has the numerical value 600 according to the Milesian system.

In ancient times, this letter denoted an aspirated voiceless velar plosive [kʱǐː̂], which in later times was either [ç] (i-sound) or [x] (ah-sound). Similarly, the pronunciation in modern Greek is either [ç] before e- and i-sounds, or [x] before a-, o-, u-sounds and before consonants. The pronunciation of school Greek in Germany is similar. Today's pronunciation [k] for chi in some foreign words from Greek, such as chaos, choir, Christ, is thus closer to the ancient Greek pronunciation than the one usually taught in schools.

Usage

• in stochastics for a probability distribution, see chi-square distribution or for the corresponding distributed test variable, see chi-square test
• in chemistry, a (non-standard) formula symbol for the substance quantity fraction
• in set theory the indicator function
• in linear algebra for the characteristic polynomial
• In physics, material property of being polarized or magnetized in an external electric (electric susceptibility) or magnetic (magnetic susceptibility) field.
• Chi phase, metallic phase
• in the International Phonetic Alphabet, the lower case [χ] is used to denote the unvoiced uvular fricative
• in the calculation of machine elements, the related stress gradient (consideration of the supporting effect of elastically stressed near areas of stress peaks in the component, for example in the case of a notch)
• in the graph colouring for the chromatic number and the chromatic index
• as German spelling for Qi as a term for life energy, "fluidum" in traditional Chinese medicine (TCM)
• in flight dynamics as orbit azimuth
• as a symbol for Jesus Christ: The Greek X is the first letter of the name Χριστός and at the same time symbolizes the cross.

See also

• Chiasmus
• Chiasma
• optic chiasm