For other meanings of ETA, see ETA (disambig).

Eta (upper case Η, lower case η) is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8. The letter was pronounced like English long A (long E in most Continential European languages) in Ancient Greek.

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).

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 physics, the refractive index of an optical medium (although the letter n is used when Greek symbols are unavailable).
• In Statistics, the Eta² is the "partial regression coefficient".