Αα	Alpha	Νν	Nu
Ββ	Beta	Ξξ	Xi
Γγ	Gamma	Οο	Omicron
Δδ	Delta	Ππ	Pi
Εε	Epsilon	Ρρ	Rho
Ζζ	Zeta	Σσς	Sigma
Ηη	Eta	Ττ	Tau
Θθ	Theta	Υυ	Upsilon
Ιι	Iota	Φφ	Phi
Κκ	Kappa	Χχ	Chi
Λλ	Lambda	Ψψ	Psi
Μμ	Mu	Ωω	Omega

Numerals

Use as scientific symbols

Omicron (/ˈoʊmɪkrɒn, ˈɒmɪkrɒn, oʊˈmaɪkrɒn/;^[1] uppercase Ο, lowercase ο, Greek: όμικρον) is the 15th letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin:

. In classical Greek, omicron represented the close-mid back rounded vowel IPA: [o] in contrast to omega which represented the open-mid back rounded vowel IPA: [ɔː] and the digraph ου which represented the long close-mid back rounded vowel IPA: [oː]. In modern Greek, both omicron and omega represent the mid back rounded vowel IPA: [o̞] or IPA: [ɔ̝]. 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.

Use

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).

Mathematics

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 numerals

Main article: Greek numerals

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]

Astronomy

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 ( $0$ ).

Medicine

Main article: SARS-CoV-2 Omicron variant

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]

History

Detail from a fifth century BCE 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]^(p 19) 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:/.^[8]^{(pp 56, 71)}

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 $ὦ$ ).^[9]^[d] By the second and third centuries CE, 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.^[9]

Mispronunciation

Many people mispronounce Omicron as “Omnicron” due to the unfamiliarity of the letter, and the use of “Omni-“ in many words.^[11]^[12]

Character encodings

Greek omicron / Coptic O^[13]

Character information
Preview	Ο		ο		Ⲟ		ⲟ
Unicode name	GREEK CAPITAL LETTER OMICRON		GREEK SMALL LETTER OMICRON		COPTIC CAPITAL LETTER O		COPTIC SMALL LETTER O
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^[14]

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
Unicode name	MATHEMATICAL BOLD CAPITAL OMICRON		MATHEMATICAL BOLD SMALL OMICRON		MATHEMATICAL ITALIC CAPITAL OMICRON		MATHEMATICAL ITALIC SMALL OMICRON		MATHEMATICAL BOLD ITALIC CAPITAL OMICRON		MATHEMATICAL BOLD ITALIC SMALL OMICRON
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
Unicode name	MATHEMATICAL SANS-SERIF BOLD CAPITAL OMICRON		MATHEMATICAL SANS-SERIF BOLD SMALL OMICRON		MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL OMICRON		MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL OMICRON
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

Footnotes

^ 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]}
^ 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]}
^ 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.
^ This is confirmed by the text of the so-called Letter Tragedy of the fifth-century BCE comic poet Callias, and also by a passage in Plato's Cratylus, where Socrates states:
[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.^[10]

Use