Chi
Chi (Χ, χ) is the 22nd letter of the Greek alphabet, said “kye” (rhyming with sky). In statistics χ² is the chi-square distribution and the chi-square test.
How to read it
Dictionary.com, Collins and the Cambridge Dictionary all give /kaɪ/. Collins describes chi as a consonant, transliterated as ch or rarely kh. “Chi” pronounced “chee” is a different word: the vital energy of Chinese medicine and martial arts, also spelled qi (Cambridge, Collins). The capital Χ has the same shape as the Latin X (U+0058).
Also searched as: chi letter, chi symbol, χ, Χ, χ meaning.
Capital and small forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Χ | U+03A7 | GREEK CAPITAL LETTER CHI |
| Small | χ | U+03C7 | GREEK SMALL LETTER CHI |
Unicode also has a Latin small letter chi, ꭓ (U+AB53).
Chi-square in statistics
The chi-square distribution is written χ² with its degrees of freedom, df: OpenStax’s Introductory Statistics writes X ∼ χ²df, where the X is a Latin capital naming the random variable and χ is the Greek letter. Its mean is the degrees of freedom, μ = df.
In AI and machine learning
scikit-learn’s chi2 function computes chi-squared statistics between each non-negative feature and the class, and the features with the highest scores can be kept for a model. Its documentation gives word counts in document classification as an example.
Write in Greek letters
stringmash.comGreek alphabet translatorWrite English in Greek letters by sound, or turn Greek back into Latin letters as Greek passports do.Read next
- Next in Greek lettersPsiPsi (Ψ, ψ) is the 23rd letter of the Greek alphabet, said “sye” or “psye”. In quantum physics Ψ and ψ are the wave function in the Schrödinger equation.
- The same shape in Smoke and CO alarm soundsLow-frequency 520 Hz alarm (sleeping rooms)A fire alarm in a sleeping room that sounds a low buzz in place of a high-pitched beep. It is the three-beep fire alarm rhythm at 520 Hz, which NFPA 72 requires of alarms provided to wake people in sleeping areas.
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- chi (Dictionary.com entry), Dictionary.com Unabridged; Collins English Dictionary, Dictionary.com, Entry, American and British (Collins) senses
- chi (Cambridge Dictionary entry), Cambridge Advanced Learner’s Dictionary, Cambridge University Press, Entry
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek capital and small letters
- C0 Controls and Basic Latin: Range 0000–007F (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Basic Latin
- Introductory Statistics 2e, 11.1 Facts About the Chi-Square Distribution, OpenStax (Rice University), Section 11.1
- sklearn.feature_selection.chi2, scikit-learn documentation, scikit-learn developers, Description
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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