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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
Greek alphabet
Α α Alpha Β β Beta Γ γ Gamma
Δ δ Delta Ε ε Epsilon Ζ ζ Zeta
Η η Eta Θ θ Theta Ι ι Iota
Κ κ Kappa Λ λ Lambda Μ μ Mu
Ν ν Nu Ξ ξ Xi Ο ο Omicron
Π π Pi Ρ ρ Rho Σ σ Sigma
Τ τ Tau Υ υ Upsilon Φ φ Phi
Χ χ Chi Ψ ψ Psi Ω ω Omega
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