Eta
Eta (uppercase Η, lowercase η; Ancient Greek: ἦτα ē̂ta [êːtaː] or Greek: ήτα ita [ˈita]) is the seventh letter of the Greek alphabet. Originally denoting a consonant /h/, its sound value in the classical Attic dialect of Ancient Greek was a long vowel [ɛː], raised to [i] in hellenistic Greek, a process known as iotacism.
Greek alphabet  



History  
Use in other languages  
Related topics  
 
In the ancient Attic number system (Harodianic or acrophonic numbers), the number 100 was represented by "Η", because it was the initial of ΗΕΚΑΤΟΝ, the ancient spelling of ἑκατόν = "one hundred". In the latter system of (Classical) Greek numerals it has a value of 8.
Eta was derived from the Phoenician letter heth
History
Consonant h
The letter shape 'H' was originally used in most Greek dialects to represent the sound /h/, a voiceless glottal fricative. In this function, it was borrowed in the 8th century BC by the Etruscan and other Old Italic alphabets, which were based on the Euboean form of the Greek alphabet. This also gave rise to the Latin alphabet with its letter H.
Other regional variants of the Greek alphabet (epichoric alphabets), in dialects that still preserved the sound /h/, employed various glyph shapes for consonantal heta side by side with the new vocalic eta for some time. In the southern Italian colonies of Heracleia and Tarentum, the letter shape was reduced to a "halfheta" lacking the right vertical stem (Ͱ). From this sign later developed the sign for rough breathing or spiritus asper, which brought back the marking of the /h/ sound into the standardized postclassical (polytonic) orthography.[1] Dionysius Thrax in the second century BC records that the letter name was still pronounced heta (ἥτα), correctly explaining this irregularity by stating "in the old days the letter Η served to stand for the rough breathing, as it still does with the Romans."[2]
Long e
In the East Ionic dialect, however, the sound /h/ disappeared by the sixth century BC, and the letter was reused initially to represent a development of a long vowel /aː/, which later merged in East Ionic with /ɛː/ instead.[3] In 403 BC, Athens took over the Ionian spelling system and with it the vocalic use of H (even though it still also had the /h/ sound itself at that time). This later became the standard orthography in all of Greece.
Itacism
During the time of postclassical Koiné Greek, the /ɛː/ sound represented by eta was raised and merged with several other formerly distinct vowels, a phenomenon called iotacism or itacism, after the new pronunciation of the letter name as ita instead of eta.
Itacism is continued into Modern Greek, where the letter name is pronounced [ˈita] and represents the sound /i/ (a close front unrounded vowel). It shares this function with several other letters (ι, υ) and digraphs (ει, οι), which are all pronounced alike. This phenomenon at large is called iotacism.
Cyrillic script
Eta was also borrowed with the sound value of [i] into the Cyrillic script, where it gave rise to the Cyrillic letter И.
Uses
Letter
In Modern Greek, due to iotacism, the letter (pronounced [ˈita]) represents a close front unrounded vowel, /i/. In Classical Greek, it represented a long openmid front unrounded vowel, /ɛː/.
Symbol
Upper case
The uppercase letter Η is used as a symbol in textual criticism for the Alexandrian texttype (from Hesychius, its oncesupposed editor).
In chemistry, the letter H as symbol of enthalpy sometimes is said to be a Greek eta, but since enthalpy comes from ἐνθάλπος, which begins in a smooth breathing and epsilon, it is more likely a Latin H for 'heat'.
In information theory the uppercase Greek letter H is used to represent the concept of entropy of a discrete random variable.
Lower case
The lowercase letter η is used as a symbol in:
 Thermodynamics, the efficiency of a Carnot heat engine, or packing fraction.
 Chemistry, the hapticity, or the number of atoms of a ligand attached to one coordination site of the metal in a coordination compound. For example, an allyl group can coordinate to palladium in the η¹ mode (only one atom of an allyl group attached to palladium) or the η³ mode (3 atoms attached to palladium).
 Optics, the electromagnetic impedance of a medium, or the quantum efficiency of detectors.
 Particle physics, to represent the η mesons.
 Experimental particle physics, η stands for pseudorapidity.
 Cosmology, η represents conformal time; dt = adη.
 Cosmology, baryon–photon ratio.
 Relativity and Quantum field theory (physics), η (with two subscripts) represents the metric tensor of Minkowski space (flat spacetime).
 Statistics, η^{2} is the "partial regression coefficient". η is the symbol for the linear predictor of a generalized linear model, and can also be used to denote the median of a population, or thresholding parameter in Sparse Partial Least Squares regression.
 Economics, η is the elasticity.
 Astronomy, the seventhbrightest (usually) star in a constellation. See Bayer designation.
 Mathematics, ηconversion, see lambda calculus
 Mathematics, the Dirichlet eta function, Dedekind eta function, and Weierstrass eta function.
 In category theory, the unit of an adjunction or monad is usually denoted η.
 Biology, a DNA polymerase found in higher eukaryotes and implicated in Translesion Synthesis.
 Neural network backpropagation, and stochastic gradient descent more generally, η stands for the learning rate.
 Telecommunications, η stands for efficiency
 Electronics, η stands for the ideality factor of a bipolar transistor, and has a value close to 1.000. It appears in contexts where the transistor is used as a temperature sensing device, e.g. the thermal "diode" transistor that is embedded within a computer's microprocessor.
 Power electronics, η stands for the efficiency of a power supply, defined as the output power divided by the input power.
 Atmospheric science, η represents absolute atmospheric vorticity.
 Rheology, η represents viscosity.
 Oceanography, η is the measurement (usually in metres) of sealevel height above or below the mean sealevel at that same location.
Character encodings
 Greek Eta, Coptic Hate
Character  Η  η  Ͱ  ͱ  Ⲏ  ⲏ  

Unicode name  GREEK CAPITAL LETTER ETA  GREEK SMALL LETTER ETA  GREEK CAPITAL LETTER HETA  GREEK SMALL LETTER HETA  COPTIC CAPITAL LETTER HATE  COPTIC SMALL LETTER HATE  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  919  U+0397  951  U+03B7  880  U+0370  881  U+0371  11406  U+2C8E  11407  U+2C8F 
UTF8  206 151  CE 97  206 183  CE B7  205 176  CD B0  205 177  CD B1  226 178 142  E2 B2 8E  226 178 143  E2 B2 8F 
Numeric character reference  Η  Η  η  η  Ͱ  Ͱ  ͱ  ͱ  Ⲏ  Ⲏ  ⲏ  ⲏ 
Named character reference  Η  η  
DOS Greek  134  86  158  9E  
DOS Greek2  170  AA  225  E1  
Windows 1253  199  C7  231  E7  
TeX  \eta 
 Mathematical Eta
Character  𝚮  𝛈  𝛨  𝜂  𝜢  𝜼  

Unicode name  MATHEMATICAL BOLD CAPITAL ETA  MATHEMATICAL BOLD SMALL ETA  MATHEMATICAL ITALIC CAPITAL ETA  MATHEMATICAL ITALIC SMALL ETA  MATHEMATICAL BOLD ITALIC CAPITAL ETA  MATHEMATICAL BOLD ITALIC SMALL ETA  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  120494  U+1D6AE  120520  U+1D6C8  120552  U+1D6E8  120578  U+1D702  120610  U+1D722  120636  U+1D73C 
UTF8  240 157 154 174  F0 9D 9A AE  240 157 155 136  F0 9D 9B 88  240 157 155 168  F0 9D 9B A8  240 157 156 130  F0 9D 9C 82  240 157 156 162  F0 9D 9C A2  240 157 156 188  F0 9D 9C BC 
UTF16  55349 57006  D835 DEAE  55349 57032  D835 DEC8  55349 57064  D835 DEE8  55349 57090  D835 DF02  55349 57122  D835 DF22  55349 57148  D835 DF3C 
Numeric character reference  𝚮  𝚮  𝛈  𝛈  𝛨  𝛨  𝜂  𝜂  𝜢  𝜢  𝜼  𝜼 
Character  𝝜  𝝶  𝞖  𝞰  

Unicode name  MATHEMATICAL SANSSERIF BOLD CAPITAL ETA  MATHEMATICAL SANSSERIF BOLD SMALL ETA  MATHEMATICAL SANSSERIF BOLD ITALIC CAPITAL ETA  MATHEMATICAL SANSSERIF BOLD ITALIC SMALL ETA  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  120668  U+1D75C  120694  U+1D776  120726  U+1D796  120752  U+1D7B0 
UTF8  240 157 157 156  F0 9D 9D 9C  240 157 157 182  F0 9D 9D B6  240 157 158 150  F0 9D 9E 96  240 157 158 176  F0 9D 9E B0 
UTF16  55349 57180  D835 DF5C  55349 57206  D835 DF76  55349 57238  D835 DF96  55349 57264  D835 DFB0 
Numeric character reference  𝝜  𝝜  𝝶  𝝶  𝞖  𝞖  𝞰  𝞰 
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.
References
Look up Η or η in Wiktionary, the free dictionary. 
 Nick Nicholas (2003), "Greek /h/" Archived 20130901 at Archive.today
 παρὰ τοῖς ἀρχαίοις ὁ τύπος τοῦ Η ἐν τύπῳ δασείας ἔκειτο, ὥσπερ καὶ νῦν <παρὰ> τοῖς Ῥωμαίοις Alfredus Hilgard (ed), "In artis Dionysianae §6" in Grammatici Graeci. Scholia in Dionysii Thracis Artem Grammaticam (1901), p. 486.
 Sihler, Andrew L. (1995). New Comparative Grammar of Greek and Latin (illustrated ed.). New York: Oxford University Press. pp. 10–20. ISBN 0195083458.