Eta (uppercase , lowercase ; grc|ἦτα ''ē̂ta'' or ell|ήτα ''ita'' ) is the seventh letter of the Greek alphabet
. Originally denoting the consonant
/h/, its sound value in the classical Attic dialect
of Ancient Greek
was a long vowel
, raised to in hellenistic Greek, a process known as iotacism
In the ancient Attic number system
(Herodianic or acrophonic numbers), the number 100 was represented by "", 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
. Letters that arose from eta include the Latin H
and the Cyrillic
thumb|Eta in the function of /h/ on an Attic red-figured calyx
, 515 BC. Amongst the depicted figures are [[Hermes and [[Hypnos]]. Inscriptions: – .]]
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
, 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 /h/ sound into the standardized post-classical (polytonic
[Nick Nicholas (2003), "Greek /h/"](_blank)
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."
In the East Ionic dialect
, however, the sound /h/ disappeared by the sixth century BC, and the letter was re-used initially to represent a development of a long vowel , which later merged in East Ionic with instead.
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.
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 sound /i/ (a close front unrounded vowel
). It shares this function with several other letters (ι
) and digraphs
(ει, οι), which are all pronounced alike. This phenomenon at large is called iotacism
Eta was also borrowed with the sound value of
into the Cyrillic script
, where it gave rise to the Cyrillic letter И
In Modern Greek
, due to iotacism
, the letter (pronounced ) represents a close front unrounded vowel
, . In Classical Greek
, it represented a long open-mid front unrounded vowel
The uppercase letter Η is used as a symbol in textual criticism
for the Alexandrian text-type
, its once-supposed editor).
, 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
In information theory the uppercase Greek letter H is used to represent the concept of entropy
of a discrete random variable.
The lowercase letter η is used as a symbol in:
, the efficiency of a Carnot heat engine
, or packing fraction
, 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).
, the electromagnetic impedance
of a medium, or the quantum efficiency
, to represent the η meson
*Experimental particle physics
, η stands for pseudorapidity
, η represents conformal time
; dt = adη.
, baryon–photon ratio
and Quantum field theory
(physics), η (with two subscripts) represents the metric tensor
of Minkowski space
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.
, ''η'' is the elasticity
, the seventh-brightest (usually) star in a constellation
. ''See'' Bayer designation
*Mathematics, η-conversion, see lambda calculus
*Mathematics, the Dirichlet eta function
, Dedekind eta function
, and Weierstrass eta function
*In category theory
, the unit of an adjunction
is usually denoted ''η''.
, 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
, η stands for efficiency
, 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.
, η stands for the efficiency of a power supply
, defined as the output power divided by the input power.
, η represents absolute atmospheric vorticity
, η represents viscosity
, η is the measurement (usually in metres) of sea-level height above or below the mean sea-level at that same location.
Greek Eta, Coptic Ayta
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.