|Use in other languages|
Omicron (/ , /,; uppercase Ο, lowercase ο, Greek: όμικρον is the 15th letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin:
In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation, 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 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.
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]
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position. 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).
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. On November 26, 2021, Omicron was assigned to the B.1.1.529 variant of concern.
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/.(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:/.(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).[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.
|Unicode name||GREEK CAPITAL LETTER OMICRON||GREEK SMALL LETTER OMICRON||COPTIC CAPITAL LETTER O||COPTIC SMALL LETTER O|
|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
|Named character reference||Ο||ο|
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.
|Unicode name||MATHEMATICAL BOLD
|MATHEMATICAL BOLD ITALIC
|MATHEMATICAL BOLD ITALIC|
|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
|Unicode name||MATHEMATICAL SANS-SERIF
BOLD CAPITAL OMICRON
BOLD SMALL OMICRON
BOLD ITALIC CAPITAL OMICRON
BOLD ITALIC SMALL OMICRON
|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