upsilon

Upsilon is the 20th letter of the Greek alphabet, written uppercase Υ and lowercase υ. It is the origin of the Latin letter Y and is widely used as a symbol in science and mathematics. The name upsilon derives from the ancient Greek word υψιλον (hypsilon), and the letter historically represented a close vowel that varied in pronunciation across dialects.

In classical Greek, upsilon represented a close front rounded vowel (/y/) or a close back (/u/) depending on dialect; in later Greek, especially in Koine and modern pronunciation, its sound shifted toward /i/ in many contexts due to iotacism.

In the system of Greek numerals, upsilon has the value 400. Its typographic forms include the standard uppercase Υ and lowercase υ, as well as the lunate variant ϒ used in some early typography.

The letter has several prominent uses. In particle physics, Υ denotes the bottomonium meson family, with states such as Υ(1S) and Υ(2S). In astronomy, upsilon is used in Bayer designations for stars within constellations, for example Upsilon Andromedae. In linguistics and mathematics, upsilon is commonly employed as a variable or parameter, and in Unicode it is encoded as U+03A5 for uppercase and U+03C5 for lowercase.