Xi
Xi (Ξ, ξ) is the 14th letter of the Greek alphabet. It stands for the ks sound and is written x in Latin letters; English speakers say it “zye”, “sye” or “ksee”.
How to read it
Dictionary.com gives three American pronunciations: “zye”, “sye” and “ksee”. The Collins English Dictionary lists four British ones, /ksaɪ, saɪ, ksiː, zaɪ/, and the Cambridge Dictionary gives just “sye” /saɪ/. Collins describes xi as a composite consonant, transliterated as x. The capital Ξ is three horizontal strokes; the “identical to” sign ≡ of mathematics (U+2261) is a different character.
Also searched as: xi letter, xi symbol, ξ, Ξ.
Capital and small forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Ξ | U+039E | GREEK CAPITAL LETTER XI |
| Small | ξ | U+03BE | GREEK SMALL LETTER XI |
Xi in physics
Ξ is the symbol for the xi baryon; baryons are particles made of three quarks. OpenStax’s University Physics lists Ξ⁺ and Ξ⁻ with a rest mass of 1315 MeV and a lifetime of 2.9 × 10⁻¹⁰ seconds, beside the Sigma (Σ) and Omega (Ω) baryons.
Why the WHO skipped Xi
In November 2021 the World Health Organization named the variant B.1.1.529 Omicron, skipping Nu and Xi. It told CNN that “Xi was not used because it is a common surname”. CNN noted that the Greek letter resembles the Chinese surname Xi, as in China’s leader Xi Jinping, though the two are pronounced differently, and that this fuelled speculation about the choice.
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 lettersOmicronOmicron (Ο, ο) is the 15th letter of the Greek alphabet, a short o. Since November 2021 it is also the World Health Organization’s name for the SARS-CoV-2 variant B.1.1.529.
- The same shape in Tire sidewall markingsSpeed rating (H in 95H)The letter at the end of a tire’s service description, such as H in 95H, is its speed rating: the speed category the tire is rated for. H is 210 km/h (130 mph).
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- xi (Dictionary.com entry), Dictionary.com Unabridged; Collins English Dictionary, Dictionary.com, Entry, American and British (Collins) senses
- xi (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
- Mathematical Operators: Range 2200–22FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Relations
- University Physics Volume 3, 11.1 Introduction to Particle Physics, OpenStax (Rice University), Table 11.1; Hadrons
- Why WHO skipped two letters of the Greek alphabet in naming Omicron, 29 November 2021, CNN, Article
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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