Omicron (letter)

Omicron (uppercase Ο, lowercase ο, literally "small o": όμικρον, o mikron, micron meaning 'small' in contrast to omega) is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. This letter is derived from the Phoenician letter ayin . In classical Greek, omicron represented the sound [o] in contrast to omega [ɔː] and ου [oː]. In modern Greek, omicron represents the mid back rounded vowel /o/. Letters that arose from omicron include Roman O and Cyrillic O.

The upper-case letter of omicron (O) was originally used in mathematics as a symbol for Big O notation (representing a function's asymptotic growth rate), but has fallen out of favor because omicron is indistinguishable from the Latin letter O and easily confused with the digit zero (0).

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

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