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Eta (uppercase , lowercase ; grc, ἦτα ''ē̂ta'' or ell, ήτα ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel . Originally denoting the voiceless glottal fricative in most dialects, its sound value in the classical Attic dialect of Ancient Greek was a long open-mid front unrounded vowel , raised to in hellenistic Greek, a process known as
iotacism Iotacism ( el, ιωτακισμός, ''iotakismos'') or itacism is the process of vowel shift by which a number of vowels and diphthongs converged towards the pronunciation in post-classical Greek and Modern Greek. The term "iotacism" refer ...
or itacism. In the ancient Attic number system (Herodianic or acrophonic numbers), the number 100 was represented by "", because it was the initial of , the ancient spelling of = "one hundred". In the later system of (Classical) Greek numerals eta represents 8. Eta was derived from the Phoenician letter heth . Letters that arose from eta include the Latin H and the Cyrillic letter И and Й.


History


Consonant h

The letter shape 'H' was originally used in most Greek dialects to represent the 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 , 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 Tarentum may refer to: * Taranto, Apulia, Italy, on the site of the ancient Roman city of Tarentum (formerly the Greek colony of Taras) **See also History of Taranto * Tarentum (Campus Martius), also Terentum, an area in or on the edge of the Camp ...
, the letter shape was reduced to a "half-heta" 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 sound into the standardized post-classical ( polytonic) orthography.Nick Nicholas (2003), "Greek "
Dionysius Thrax Dionysius Thrax ( grc-gre, Διονύσιος ὁ Θρᾷξ ''Dionýsios ho Thrâix'', 170–90 BC) was a Greek grammarian and a pupil of Aristarchus of Samothrace. He was long considered to be the author of the earliest grammatical text on the Gr ...
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."


Long e

In the East Ionic dialect, however, the sound disappeared by the sixth century BC, and the letter was re-used initially to represent a development of a long open front unrounded vowel , which later merged in East Ionic with the long open-mid front unrounded vowel instead. In 403 BC,
Athens Athens ( ; el, Αθήνα, Athína ; grc, Ἀθῆναι, Athênai (pl.) ) is both the capital and largest city of Greece. With a population close to four million, it is also the seventh largest city in the European Union. Athens dominates a ...
took over the Ionian spelling system and with it the vocalic use of H (even though it still also had the sound itself at that time). This later became the standard orthography in all of Greece.


Itacism

During the time of post-classical Koiné Greek, the 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 and represents the close front unrounded vowel . It shares this function with several other letters ( ι, υ) and digraphs (ει, οι), which are all pronounced alike.


Cyrillic script

Eta was also borrowed with the sound value of into the
Cyrillic script The Cyrillic script ( ), Slavonic script or the Slavic script, is a writing system used for various languages across Eurasia. It is the designated national script in various Slavic, Turkic, Mongolic, Uralic, Caucasian and Iranic-speaking c ...
, where it gave rise to the Cyrillic letter И.


Uses


Letter

In Modern Greek, due to
iotacism Iotacism ( el, ιωτακισμός, ''iotakismos'') or itacism is the process of vowel shift by which a number of vowels and diphthongs converged towards the pronunciation in post-classical Greek and Modern Greek. The term "iotacism" refer ...
, the letter (pronounced ) represents a close front unrounded vowel, . In Classical Greek, it represented the long open-mid front unrounded vowel .


Symbol


Upper case

The uppercase letter Η is used as a symbol in textual criticism for the Alexandrian text-type (from Hesychius, its once-supposed editor). In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, 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 Η 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. * Aeronautics, the propulsive efficiency, or percentage of chemical energy converted to kinetic energy. *
Chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, 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 In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . ...
can coordinate to
palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...
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 In experimental particle physics, pseudorapidity, \eta, is a commonly used spatial coordinate describing the angle of a particle relative to the beam axis. It is defined as :\eta \equiv -\ln\left tan\left(\frac\right)\right where \theta is the ...
. * 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 Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, indust ...
, ''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 Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, the seventh-brightest (usually) star in a constellation. ''See'' Bayer designation. *Mathematics, η-reduction in 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 Stochastic gradient descent (often abbreviated SGD) is an iterative method for optimizing an objective function with suitable smoothness properties (e.g. differentiable or subdifferentiable). It can be regarded as a stochastic approximation of ...
more generally, η stands for the
learning rate In machine learning and statistics, the learning rate is a tuning parameter in an optimization algorithm that determines the step size at each iteration while moving toward a minimum of a loss function. Since it influences to what extent newly ...
. * 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 A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As ...
, defined as the output power divided by the input power. * Atmospheric science, η represents absolute atmospheric vorticity. * Rheology, η represents
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
. * Oceanography, η is the measurement (usually in metres) of sea-level height above or below the mean sea-level at that same location.


Character encodings


Greek Eta, Coptic Ayta


Mathematical Eta

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.


See also

*
Hurricane Eta Hurricane Eta was a deadly and erratic Category 4 hurricane that devastated parts of Central America in early November 2020. The record-tying twenty-eighth named storm, thirteenth hurricane, and sixth major hurricane of the extremely-active 2 ...


References

{{Reflist Greek letters Vowel letters