Eta (letter)
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- For other uses, see Eta.
Eta (uppercase Η, lowercase η) is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8. It was derived from the Phoenician letter Heth . Letters that arose from Eta include the Latin H and the Cyrillic letter И.
In Modern Greek the letter is pronounced {{IPA. In Classical Greek, it was pronounced as a long open-mid front unrounded vowel, [ɛː]. The symbol was initially used to denote the rough breathing, {{IPA. In the Ionic dialect, where this sound had disappeared by the sixth century BC, the letter was used to represent the long [ɛː]. When the Ionic alphabet was adopted in Athens in 403 BC, [ɛː] (previously written as Ε) was also written as Η there too, hence the modern usage.
Symbol
Eta is sometimes used in place of eng (ŋ) when eng is not available, because of their similar appearance.
The upper-case letter Η is used as a symbol for:
- In textual criticism, the Alexandrian text-type (from Hesychius, its once-supposed editor).
- In chemistry, enthalpy.
- In thermodynamics, the efficiency of a Carnot heat engine.
- In chemistry, the number of electrons shared between a metal center and a ligand in a coordination compound. For example, an allyl group can coordinate to palladium in the η¹ mode or the η³ mode.
- In optics, the refractive index of an optical medium (although the letter n is more common).
- In particle physics, to represent the η baryons.
- In statistics, η2 is the "partial regression coefficient".
- In astronomy, the seventh brightest (usually) star in a constellation. See Bayer designation.
- In experimental particle physics, η stands for pseudorapidity.
- In mathematics, η-conversion, see lambda calculus
- In fluid dynamics, η stands for viscosity.
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