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.
More greek letters
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
Last reviewed September 30, 2026 by Rory Hansen. How we research