|Archaic local variants|
|In other languages|
Eta (uppercase Η, lowercase η; Greek: Ήτα Ēta) 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 medieval Greek, a process known as iotacism.
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 ultimately gave rise to the Latin alphabet with its letter H.
Long e 
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 /aː/, which later merged in East Ionic with /ɛː/ 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.
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. One of them was a tack-like shape, looking like the left half of an H. This system was first used in the southern Italian colonies of Heracleia and Tarentum. When Greek orthography was codified by grammarians in the Hellenistic era, they used a diacritic symbol derived from this half-H shape to signal the presence of /h/, and added as its counterpart a reverse-shaped diacritic to denote absence of /h/. These symbols were the origin of the rough breathing and smooth breathing diacritics that became part of classical Greek orthography. The tack symbol has been reintroduced into modern scholarly representation of archaic Greek writing under the name of Heta.
During the time of post-classical Koiné Greek, the /ɛː/ sound represented by eta was raised and merged with several other formerly distinct vowels (iotacism). Thus in Modern Greek, Eta 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 (see iotacism).
Cyrillic script 
Upper case 
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'.
Lower case 
The lower-case letter η is used as a symbol in:
- Thermodynamics, the efficiency of a Carnot heat engine.
- 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.
- Relativity and Quantum field theory (physics), η (with two subscripts) represents the metric tensor of Minkowski space (flat spacetime).
- Statistics, η2 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.
- Economics, η is the elasticity.
- Astronomy, 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.
- Biology, a DNA polymerase found in higher eukaryotes and implicated in Translesion Synthesis.
- Neural network backpropagation, η stands for the learning rate.
- Telecommunications, Eta 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 sea-level height above or below the mean sea-level at that same location.
Character Encodings 
- Greek Eta / Coptic Hate
|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|
|UTF-8||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|
- Mathematical Eta
|Unicode name||MATHEMATICAL BOLD
|MATHEMATICAL BOLD ITALIC
|MATHEMATICAL BOLD ITALIC
|UTF-8||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|
|UTF-16||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|
|Unicode name||MATHEMATICAL SANS-SERIF
BOLD CAPITAL ETA
BOLD SMALL ETA
BOLD ITALIC CAPITAL ETA
BOLD ITALIC SMALL ETA
|UTF-8||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|
|UTF-16||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.