Upsilon Symbol (υ): Meaning, Uses and How to Type It

Upsilon is the twentieth letter of the Greek alphabet. It is rarely used as a math variable, because the lowercase υ is easily confused with a Latin v and with nu (ν), but the capital has two well-known jobs in science: it names the Upsilon meson of particle physics and the mass-to-light ratio in astronomy. Upsilon is also the ancestor of four Latin letters, Y, U, V and W.

This guide explains what upsilon means and how to type υ, Υ and the hooked ϒ on any device, in HTML and in LaTeX.

Upsilon Definition

Upsilon (noun): the twentieth letter of the Greek alphabet, written Υ (capital) and υ (small). English speakers say "UP-sih-lon" or "OOP-sih-lon" (American) and "yoop-SY-lən" (British); in modern Greek it is "EE-psee-lon." It comes after tau (τ) and before phi (φ) and stood for 400 in the Greek numeral system.

The name means "simple u" (u psilon), coined by Byzantine grammarians to tell the letter apart from the diphthong οι, which had come to sound the same. The Romans borrowed the letter twice: once early, as V (which later split into U, V and W), and again in the first century BC as Y, to spell Greek loanwords. For all 24 letters with their LaTeX commands, see the Greek alphabet in LaTeX lesson, and for how each letter is used as a math symbol, see Greek letters as math symbols.

Upsilon Symbol (υ): Copy and Paste

Copy all three forms from the box below (use the copy button), or take the code you need from the table:

1υ Υ ϒ
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The hooked form ϒ (GREEK UPSILON WITH HOOK SYMBOL) exists because physicists and mathematicians want a capital upsilon that cannot be mistaken for a Latin Y. LaTeX's \Upsilon already draws the capital with curled arms for the same reason.

What the Upsilon Symbol Means

The Upsilon Meson in Particle Physics

The Υ meson (also written ϒ) is a bound state of a bottom quark and a bottom antiquark, a kind of particle called bottomonium. It was discovered in 1977 at Fermilab by an experiment led by Leon Lederman, and it was the first evidence for the bottom (b) quark, the fifth of the six quarks. The ground state Υ(1S) has a mass of about 9.46 GeV/c², roughly ten times the mass of a proton, and its excited states are written Υ(2S), Υ(3S) and so on.

Mass-to-Light Ratio in Astronomy

Astronomers use Υ for the mass-to-light ratio, the mass of a star, galaxy or cluster divided by its luminosity, usually quoted in solar units (Υ☉ = 1 for the Sun). Galaxy clusters show values in the hundreds, far more mass than their stars account for, which is one of the classic pieces of evidence for dark matter.

The Upsilon Vowel in Phonetics

The International Phonetic Alphabet uses a letter based on upsilon, ʊ (LATIN SMALL LETTER UPSILON, U+028A), for the short vowel in English "foot," "book" and "put." It is a separate Unicode character from the Greek υ, so linguists should copy ʊ rather than the Greek letter.

The Pythagorean Letter

Ancient and Renaissance writers called upsilon "the letter of Pythagoras," reading its Y shape as a fork in the road of life: one branch leading to virtue, the other to vice.

How to Type Upsilon

Windows: upsilon has no short Alt code in the old IBM character set. In Word, type 03C5 and press Alt + X; elsewhere, open Character Map and search "upsilon."

Mac: there is no Option-key shortcut. Add the Greek input source in System Settings > Keyboard > Input Sources and press Y for υ or Shift + Y for Υ, or open the Character Viewer with Control + Command + Space and search "upsilon."

iPhone and Android: add the Greek keyboard (Settings > General > Keyboard > Keyboards on iOS, or Gboard's language settings), switch with the globe key and tap Y.

Microsoft Word: type 03C5 and press Alt + X (03A5 for Υ, 03D2 for ϒ), or use Insert > Symbol > More Symbols with the Greek and Coptic subset. Inside a Word equation (Alt + =), \upsilon followed by a space also works.

Google Docs: go to Insert > Special characters and search "upsilon," or open Insert > Equation and type \upsilon followed by a space.

HTML: write υ for υ, Υ for Υ and ϒ for ϒ, or the numeric forms υ, Υ and ϒ.

Upsilon in LaTeX

Use \upsilon for υ and \Upsilon for Υ in math mode; with amsmath, \varUpsilon gives an italic capital. Unlike alpha, beta or nu, upsilon does have a capital command, because Computer Modern draws Υ with curled arms that keep it distinct from Y.

A complete example you can compile:

1\documentclass{article}
2\usepackage{amsmath}
3\begin{document}
4The $\Upsilon(1S)$ meson has a mass of about $9.46\ \mathrm{GeV}/c^2$.
5\[
6  \Upsilon_\star = \frac{M}{L} \quad \text{in units of } \Upsilon_\odot
7\]
8Lowercase upsilon is $\upsilon$; an italic capital is $\varUpsilon$.
9\end{document}
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Related symbols and guides

• Upsilon symbol page — copy the symbol and its LaTeX command
• Greek letters reference — every symbol in the category with its LaTeX command
• Greek alphabet in LaTeX — how to typeset these symbols in a document
• Tau symbol (τ)
• Phi symbol (Φ φ)

Upsilon Symbol FAQ

What is the upsilon symbol?

The upsilon symbol is the Greek letter upsilon: υ in lowercase (U+03C5) and Υ in uppercase (U+03A5), with a hooked capital ϒ (U+03D2) for scientific use. It is the twentieth letter of the Greek alphabet and names the Upsilon meson and the mass-to-light ratio.

How do you pronounce upsilon?

In American English "UP-sih-lon" or "OOP-sih-lon"; in British English "yoop-SY-lən." Modern Greek speakers say "EE-psee-lon," because the letter is now pronounced like the i in "machine."

What is the difference between υ, ν and v?

υ is upsilon (U+03C5), with a round bottom; ν is nu (U+03BD), with a pointed bottom like a v; and v is the Latin letter. They look alike in many italic fonts, so use the LaTeX commands \upsilon and \nu rather than typing v.

How do you type the upsilon symbol?

In Word type 03C5 and press Alt + X. On a Mac or phone, add the Greek keyboard and press Y. In HTML write υ, and in LaTeX write \upsilon or \Upsilon. Or copy υ from the box at the top of this page.

Is Υ the same as the letter Y?

They share an ancestor and often look identical, but they are different characters: Greek capital upsilon is U+03A5 and Latin Y is U+0059. Use the Greek letter (or ϒ) for the Upsilon meson so the symbol stays searchable and typographically correct.

Writing with Greek Symbols

Particle physics tables, astronomy papers and phonetics notes each need the right upsilon, typeset consistently and never swapped for a Y or a v. Octree is an AI-powered LaTeX editor that turns plain-language descriptions into properly formatted equations. Create professional documents at useoctree.com.