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).
  
• 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 physics, the refractive index of an optical medium (although the letter n is used when Greek symbols are unavailable).
  
• 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