What does chi mean?
Chi is the twenty-second letter of the Greek alphabet, written Χ (uppercase) and χ (lowercase) and pronounced to rhyme with 'sky'. It appears frequently in mathematics and science, most notably in the chi-square test used in statistics to assess how observed data compare with expected results. Separately, chi is an English rendering of the Chinese concept of vital energy or life force — more commonly romanized today as qi — which in traditional Chinese medicine and martial arts such as tai chi is believed to circulate through the body and govern health and wellbeing. In this sense it is pronounced 'chee'. The two meanings are entirely unrelated in origin but coexist comfortably in English. Writers should choose pronunciation and spelling according to context: the Greek letter takes /kaɪ/, while the Eastern philosophical concept takes /tʃiː/, making chi one of those rare English words with two well-established pronunciations attached to distinct senses.
The twenty-second letter of the Greek alphabet (Χ, χ), transliterated into English as kh or ch.
"In statistics, the chi-square test uses the Greek letter chi in its name."
Pronounced /kaɪ/, rhyming with 'sky'.
The vital life force or energy believed in traditional Chinese philosophy and medicine to flow through all living things; now more commonly spelled qi.
"Acupuncturists aim to restore the smooth flow of chi along the body's meridians."
Pronounced /tʃiː/ ('chee'); the pinyin spelling 'qi' is now preferred in most modern publications.
Rarely pluralized; the plural 'chis' would refer only to multiple instances of the Greek letter itself, not to the uncountable concept of vital energy.
"The page was decorated with rows of chis and other Greek letters."
The Greek letter chi looks exactly like our letter X — which is why 'Xmas' is really just an abbreviation built on chi, the first letter of Christos.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chi
As the name of a Greek letter, chi comes directly from ancient Greek Χῖ, the twenty-second letter of the alphabet, which passed into Latin and then into English. The lowercase form χ has been adopted across mathematics and the sciences, giving rise to terms such as 'chi-square' in statistics. The second sense — vital energy — derives independently from the Mandarin Chinese qì (氣), meaning breath or energy, entering English through Wade-Giles romanization as 'chi' during the twentieth century as interest in Chinese medicine and martial arts grew. Related English words formed on the Greek letter include 'chiasmus' and 'chiastic', which reference the crossing shape of χ.
Related word forms
How chi is actually used
When denoting the Greek letter, chi rhymes with 'sky' (/kaɪ/); when referring to the Chinese concept of vital energy, it is usually pronounced 'chee' (/tʃiː/) and increasingly written as 'qi'. The compound 'tai chi' refers to the Chinese martial art and exercise form.
Easily confused with chi
Qi is simply the standard modern romanization of the same Chinese word that 'chi' renders in Wade-Giles; they refer to identical concepts.
Chai is a spiced tea drink, whereas chi is either a Greek letter or a term for vital energy.