In Modern Greek the letter is pronounced . In Classical Greek, it was pronounced as a long . The symbol was initially used to denote the rough breathing, . In the Ionic dialect, where this sound had disappeared by the sixth century BC, the letter was used to represent the long When the Ionic alphabet was adopted in Athens in 403 BC, [ (previously written as Ε) was also written as Η there too, hence the modern usage.

Symbol

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 for:

• In textual criticism, the Alexandrian text-type (from Hesychius, its once-supposed editor).
• In chemistry, enthalpy.

The lower-case letter η is used as a symbol for:

• In thermodynamics, the efficiency of a Carnot heat engine.
• In chemistry, 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.
• In optics, the refractive index of an optical medium (although the letter n is more common).
• In particle physics, to represent the η baryons.
• In statistics, η² is the "partial regression coefficient".
• In astronomy, the seventh brightest (usually) star in a constellation. See Bayer designation.
• In experimental particle physics, η stands for pseudorapidity.
• In mathematics, η-conversion, see lambda calculus
• In fluid dynamics, η stands for viscosity.

Greek letters | Vowel letters

Η | Eta (lizherenn) | Ета | Eta | Eta | Eta | Ήτα | Η | Eta (greko) | Êta | Éite | Eta | Η | Eta (lettera) | אטא | Èta | Η | Eta (bokstav) | Eta | Eta | Η | Eeta | Eta | Η