Eta  (uppercase ?, lowercase ?; Ancient Greek: ta [:ta] or Greek: ita ['ita]) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel IPA: [i]. Originally denoting the voiceless glottal fricative IPA: [h] in most dialects, its sound value in the classical Attic dialect of Ancient Greek was a long open-mid front unrounded vowel IPA: [?:], raised to IPA: [i] in hellenistic Greek, a process known as iotacism 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.
The letter shape 'H' was originally used in most Greek dialects to represent the voiceless glottal fricative IPA: [h]. 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 IPA: [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 "half-heta" lacking the right vertical stem (?). From this sign later developed the sign for rough breathing or spiritus asper, which brought back the marking of the IPA: [h] sound into the standardized post-classical (polytonic) orthography. Dionysius Thrax in the second century BC records that the letter name was still pronounced heta (), correctly explaining this irregularity by stating "in the old days the letter ? served to stand for the rough breathing, as it still does with the Romans."
In the East Ionic dialect, however, the sound IPA: [h] disappeared by the sixth century BC, and the letter was re-used initially to represent a development of a long open front unrounded vowel IPA: [a:], which later merged in East Ionic with the long open-mid front unrounded vowel IPA: [?:] 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 IPA: [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 IPA: [?:] 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 ['ita] and represents the close front unrounded vowel IPA: [i]. It shares this function with several other letters (?, ?) and digraphs (, ), which are all pronounced alike.
In Modern Greek, due to iotacism, the letter (pronounced ['ita]) represents a close front unrounded vowel, IPA: [i]. In Classical Greek, it represented the long open-mid front unrounded vowel IPA: [?:].
In chemistry, the letter H as symbol of enthalpy sometimes is said to be a Greek eta, but since enthalpy comes from , 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.
The lowercase letter ? is used as a symbol in:
|Unicode name||GREEK CAPITAL LETTER ETA||GREEK SMALL LETTER ETA||GREEK CAPITAL LETTER HETA||GREEK SMALL LETTER HETA||COPTIC CAPITAL LETTER AYTA||COPTIC SMALL LETTER AYTA|
|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
|Named character reference||Η||η|
|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
|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
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.