Xi ( or ; uppercase Ξ, lowercase ξ; ) is the fourteenth letter of the Greek alphabet, representing the voiceless consonant cluster. Its name is pronounced in Modern Greek. In the system of Greek numerals, it has a value of 60. Xi was derived from the Phoenician letter samekh. Xi is distinct from the letter chi, which gave its form to the Latin letter X.

Greek

Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the consonant cluster /ks/. In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the dialects of most of the Greek mainland and Euboea, the cluster /ks/ was represented by Χ (which in classical Greek is chi, used for ). Because this variant of the Greek alphabet was used in Magna Graecia (the Greek colonies in Sicily and the southern part of the Italian peninsula), the Latin alphabet borrowed Χ rather than Ξ as the Latin letter that represented the /ks/ cluster that was also present in Latin.

Cyrillic

The Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).

Mathematics and science

Uppercase

The uppercase letter Ξ is used as a symbol in various contexts.

Pure mathematics

Physics

Other uses

Lowercase

The lowercase letter ξ is used as a symbol for:

Pure mathematics

Physics and astronomy

Other uses

Other uses

Uppercase Ξ is used as an 'E' to stylise company names/logos like Razer (styled as RΛZΞR), Tesla (styled as TΞSLA), the presidential campaigns of Joe Biden (styled as BIDΞN), musician Banners (styled as BANNΞRS), and in South Korean boy group ZE:A's newest logo (styled as "ZΞA") (Compare: Heavy Metal umlaut; Faux Cyrillic)

Unicode

Unicode Code Charts: Greek and Coptic (Range: 0370-03FF) The following characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style: