Eta (letter)

From Freepedia

For other uses, see Eta.

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
obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi

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 [ɛː] in Ancient Greek but [i] in Modern 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 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