Omicron
Omicron is the fifteenth 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 in contrast to omega, which represented the open-mid back rounded vowel, and the digraph which represented the long close back rounded vowel. In modern Greek, both omicron and omega represent the mid back rounded vowel. Letters that arose from omicron include Roman O and Cyrillic O and Ю. The name of the letter was originally οὖ, but it was later changed to in the Middle Ages to distinguish the letter from omega, whose name means 'big o', as both letters had come to be pronounced. In modern Greek, its name has fused into όμικρον. In the system of Greek numerals, it has a value of 70.
Use
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 are indistinguishable from the Latin letter "o" and difficult to distinguish from the Arabic numeral "zero".Mathematics
The big-O symbol was introduced by Paul Bachmann in 1894 and popularized by Edmund Landau in 1909, originally standing for "order of" 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 Omega. Neither Bachmann nor Landau ever call it "Omicron", and the word "Omicron" appears just once in the title of Knuth's paper.Greek numerals
There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron 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.
Astronomy
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 Almagest, tables of sexagesimal numbers are represented in the conventional manner for Greek numbers:. Since the letter omicron is not used in sexagesimal, it is repurposed to represent an empty number cell. In some copies, zero cells were just left blank, 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 is not a mistake, the cell is actually supposed to be empty." By coincidence, the ancient zero-value omicron resembles a modern Hindu-Arabic zero .
Medicine
The World Health Organization 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.History
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/. 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:/.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: . 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 for O for Ω were introduced.