Eta (uppercase Η, lowercase η; Ήτα) is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8. Letters that arose from Eta include the Latin H and the Cyrillic letter И.
In Modern Greek the letter, pronounced [ˈita], represents a close front unrounded vowel, /i/. In Classical Greek, it represented a long open-mid front unrounded vowel, /ɛː/.
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
In the Ionic dialect, however, the sound /h/ disappeared by the sixth century BC, and the letter was re-used to represent the long vowel /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.
In the typographic conventions of modern epigraphy, the archaic consonantal Heta (Ͱ, ͱ) in ancient inscriptions is usually rendered by means of a lowercase Latin letter h. Sometimes, symbols directly representing the old half-H glyph shape are also used. The Unicode standard of computer encoding, as of 2007, is expected to introduce code points designed for this usage in its version 5.1. Like other archaic letters, they come in a lowercase and uppercase variant (U+0370 and U+0371), to cater for the needs of modern typography.
During the time of post-classical Koiné Greek, the /ɛː/ sound represented by Eta was raised and merged with several other formerly distinct vowels. 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 (ioticism).
Eta was also borrowed with the sound value of [i] into the Cyrillic alphabet, where it gave rise to the Cyrillic letter И.
Small 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 in:
- Textual criticism, the Alexandrian text-type (from Hesychius, its once-supposed editor).
- Chemistry, enthalpy. H as symbol of enthalpy sometimes is said to be a Greek eta, but since enthalpy comes from ἐνθάλπος (smooth breathing and epsilon), it is more likely a Latin H for 'heat'.
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 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.
- Optics, the refractive index of an optical medium (although the letter n is more common). It may also refer to the electromagnetic impedance of a medium.
- Particle physics, to represent the η mesons.
- Quantum Field Theory (physics), to represent the metric tensor.
- 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.
- Experimental particle physics, η stands for pseudorapidity.
- Mathematics, η-conversion, see lambda calculus
- Fluid dynamics, η stands for viscosity.
- 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.