Omicron (/ˈmɪkrɒn, ˈɒmɪkrɒn, ˈmkrɒn/;[1] uppercase Ο, lowercase ο, όμικρον) is the 15th letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin: Phoenician ayin.svg. In classical Greek, omicron represented the sound [o] in contrast to omega [ɔː] and the digraph ου [oː]. In modern Greek, both omicron and omega represent the mid back rounded vowel /o̞/ . Letters that arose from omicron include Roman O and Cyrillic O. The word literally means "little O" (o mikron) as opposed to "great O" (ō mega).[2] In the system of Greek numerals, omicron has a value of 70.


In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation,[citation needed] but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from the Latin letter "o" (O o) and difficult to distinguish from the Arabic numeral "zero" (0).


The big-O symbol introduced by Paul Bachmann in 1894 and popularized by Edmund Landau in 1909, originally standing for "order of" ("Ordnung") and being thus a Latin letter, was apparently viewed by Donald Knuth in 1976[3] as a capital Omicron, probably in reference to his definition of the symbol (capital) Omega. Neither Bachmann nor Landau ever call it "Omicron", and the word "Omicron" appears just once in Knuth's paper: in the title.

Greek numeralsEdit

There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.

More generally, the letter omicron is used to mark the fifteenth ordinal position in any Greek-alphabet marked list. So, for example, in Euclid's Elements, when various points in a geometric diagram are marked with letters, it is effectively the same as marking them with numbers, each letter representing the number of its place in the standard alphabet.[a][b]


Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.[4][5] Such stars include Omicron Andromedae, Omicron Ceti, and Omicron Persei.

In Claudius Ptolemy's (c. 100–170) Almagest, tables of sexagesimal numbers  1 ... 59  are represented in the conventional manner for Greek numbers:[c] ′α ′β ... ′νη ′νθ  . Since the letter omicron [which represents 70 (′ο) in the standard system] is not used in sexagesimal, it is re-purposed to represent an empty number cell. In some copies, zero cells were just left blank (nothing there, value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, for the same reason that blank cells in modern tables are sometimes filled-in with a long dash (—). Both an omicron and a dash imply that "this isn't a mistake, the cell is actually supposed to be empty". By coincidence, the ancient zero-value omicron (′ο) resembles a modern Hindu-Arabic zero (0).


The World Health Organization (WHO) uses the Greek alphabet to describe variants of concern of SARS‑CoV‑2, the virus which causes COVID-19.[6] On November 26, 2021, Omicron was assigned to the B.1.1.529 variant of concern.[7]


Detail from a fifth century BC inscription of Draco's law on homicide, showing the use of O rather than Ω in the phrase "ΠΡΟΤΟΣ ΑΧΣΟΝ" (πρώτος ἄξων, "first axon")

In the earliest Greek inscriptions, only five vowel letters A E I O Y were used. Vowel length was undifferentiated, with O representing both the short vowel /o/ and the long vowels /o:/ and /ɔː/.[8] Later, in classical Attic Greek orthography, the three vowels were represented differently, with O representing short /o/, the new letter Ω representing long /ɔː/, and the so-called "spurious diphthong" OY representing long /o:/.[9]

Although the Greeks took the character O from the Phoenician letter `ayin, they did not borrow its Phoenician name. Instead, the name of the letter O in classical Attic times was simply the long version of its characteric sound: οὖ (pronounced /o:/) (that of Ω was likewise ὦ).[10] This is confirmed by the text of the so-called Letter Tragedy of the fifth-century BC comic poet Callias, and also by a passage in Plato's Cratylus, where Socrates states:[11]

[W]hen we speak of the letters of the alphabet, you know, we speak their names, not merely the letters themselves, except in the case of four: E, Y, O, and Ω


By the second and third centuries AD, distinctions between long and short vowels began to disappear in pronunciation, leading to confusion between O and Ω in spelling. It was at this time that the new names of ὂ μικρόν ("small O") for O ὦ μέγα ("great O") for Ω were introduced.[10]

Character encodingsEdit

  • Greek omicron / Coptic O[12]
Character information
Preview Ο ο
Encodings decimal hex dec hex dec hex dec hex
Unicode 927 U+039F 959 U+03BF 11422 U+2C9E 11423 U+2C9F
UTF-8 206 159 CE 9F 206 191 CE BF 226 178 158 E2 B2 9E 226 178 159 E2 B2 9F
Numeric character reference Ο Ο ο ο Ⲟ Ⲟ ⲟ ⲟ
Named character reference Ο ο
DOS Greek 142 8E 166 A6
DOS Greek-2 190 BE 233 E9
Windows 1253 207 CF 239 EF
  • Mathematical omicron[13]

These 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.

Character information
Preview 𝚶 𝛐 𝛰 𝜊 𝜪 𝝄
Encodings decimal hex dec hex dec hex dec hex dec hex dec hex
Unicode 120502 U+1D6B6 120528 U+1D6D0 120560 U+1D6F0 120586 U+1D70A 120618 U+1D72A 120644 U+1D744
UTF-8 240 157 154 182 F0 9D 9A B6 240 157 155 144 F0 9D 9B 90 240 157 155 176 F0 9D 9B B0 240 157 156 138 F0 9D 9C 8A 240 157 156 170 F0 9D 9C AA 240 157 157 132 F0 9D 9D 84
UTF-16 55349 57014 D835 DEB6 55349 57040 D835 DED0 55349 57072 D835 DEF0 55349 57098 D835 DF0A 55349 57130 D835 DF2A 55349 57156 D835 DF44
Numeric character reference 𝚶 𝚶 𝛐 𝛐 𝛰 𝛰 𝜊 𝜊 𝜪 𝜪 𝝄 𝝄

Character information
Preview 𝝤 𝝾 𝞞 𝞸
Encodings decimal hex dec hex dec hex dec hex
Unicode 120676 U+1D764 120702 U+1D77E 120734 U+1D79E 120760 U+1D7B8
UTF-8 240 157 157 164 F0 9D 9D A4 240 157 157 190 F0 9D 9D BE 240 157 158 158 F0 9D 9E 9E 240 157 158 184 F0 9D 9E B8
UTF-16 55349 57188 D835 DF64 55349 57214 D835 DF7E 55349 57246 D835 DF9E 55349 57272 D835 DFB8
Numeric character reference 𝝤 𝝤 𝝾 𝝾 𝞞 𝞞 𝞸 𝞸

Explanatory footnotesEdit

  1. ^ Greek letters-as-numbers used an older Greek alphabet with three more otherwise unused letters, two of them re‑instated in their old locations, early in the alphabet. So positions higher than 5th place (ε) were shifted from the standard alphabet; 5th place was marked with normal fifth letter epsilon (ε). The 6th letter in the conventional alphabet, that normally follows ε is ζ (zeta) but the number 6 was represented a revived ancient letter ϝ (digamma), followed by ζ which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters after ζ were likewise shifted up one place, until the second ancient letter koppa, (ϙ), was reached; it fell between π and ρ. Every letter from ρ to ω was shifted two places past its conventional ordinal position. Last place coming right after omega (ω, 800) was sampi (ϡ) which represented 900. (From that point, the system restarted, with a new tick-mark, at ͵α. The tick-mark was put in a different place (͵α rather than α) to show that the letter represented a multiple of 1,000 rather than 1.)[citation needed]
  2. ^ From Euclid up to the 19th century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers),[citation needed] whereas in modern mathematical and scientific diagrams, it is much more common to choose for markers letters that might remind readers of the word used to describe the item in question.[citation needed] For example, Feynman diagrams in particle physics label the positions of particles with the first letter of their name, either in the Latin or Greek alphabet. So  p, n, and e , represent the position on a diagram of a  proton, neutron, and electron,  respectively. The neutrino is represented by ν (Greek "nu"), since the Latin letter "n" is reserved for the neutron.[citation needed]
  3. ^ Sexagesimal Greek numbers in the Almagest are conventional:   1 2 3 4 5 6 7 8 9  =  ′α ′β ′γ ′δ ′ε ′ϝ ′ζ ′η ′θ   and   10 20 30 40 50  =  ′ι κ ′λ ′μ ′ν . Notice that ancient digamma (ϝ) is used for  6  instead of zeta (ζ, which is used for 7) . Adjacent number-letters add, so all the other numbers are made by letter pairs, such as  29 30 31  =  ′κθ ′λ ′λα . The number 59 (′νθ) is the largest value used in any single number cell in sexagesimal. That leaves xi (ξ) and the letters following it ( ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ ) free for other use: Ptolemy picked  ′ο , which normally was used for 70, to mark empty (zero) cells, perhaps because the word for "nothing", οὐδέν starts with an omicron.


  1. ^ "omicron". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ The Greek Alphabet
  3. ^ Knuth, Donald (April–June 1976). "Big Omicron and big Omega and big Theta" (PDF). SIGACT News. 8 (2): 18–24. doi:10.1145/1008328.1008329. S2CID 5230246.
  4. ^ Martin, Martha Evans (1907). The Friendly Stars (1st ed.). New York: Harper & Brothers Publishers. p. 135. Retrieved 8 February 2016.
  5. ^ Wilk, Stephen R. (2007). Medusa: Solving the Mystery of the Gorgon (1st ed.). New York; London: Oxford University Press. p. 201. ISBN 9780199887736. Retrieved 8 February 2016.
  6. ^ "Embrace the WHO's new naming system for coronavirus variants". Nature. 594 (7862): 149. 2021-06-09. Bibcode:2021Natur.594..149.. doi:10.1038/d41586-021-01508-8. PMID 34108702. S2CID 235395073.
  7. ^ "Classification of Omicron (B.1.1.529): SARS-CoV-2 Variant of Concern". World Health Organization. Retrieved 26 November 2021.
  8. ^ Sihler, Andrew (1995). New Comparative Grammar of Greek and Latin. New York: Oxford University Press. p. 19. ISBN 978-0-19-537336-3.
  9. ^ Sihler (1995), pp. 56, 71
  10. ^ a b Allen, W. Sidney (1987). Vox Graeca (third ed.). Cambridge University Press. p. 172. ISBN 978-0-521-33555-3.
  11. ^ Plato, Cratylus 393
  12. ^ "Greek and Coptic (Range: 0370–03FF)" (PDF). The Unicode Standard, Ver. 8.0. Unicode, Inc. 2015. Retrieved 8 February 2016.
  13. ^ "Mathematical Alphanumeric Symbols (Range: 1D400–1D7FF)" (PDF). The Unicode Standard, Ver. 8.0. Unicode, Inc. 2015. Retrieved 8 February 2016.

External linksEdit

  •   The dictionary definition of Ο at Wiktionary
  •   The dictionary definition of ο at Wiktionary