Eta
Eta (Η, η) is the seventh letter of the Greek alphabet. Its capital looks like the Latin H. In physics η is viscosity, and in machine learning it is a common symbol for the learning rate.
How to read it
Eta is said EE-tuh in Britain and AY-tuh in America (Cambridge Dictionary: /ˈiː.tə/ and /ˈeɪ.t̬ə/). In Modern Greek ήτα, “Eeta”, sounds like the ee in “feet”, the same sound as Iota; the University of Arizona’s guide counts five ways of spelling that sound in Greek. ETA in capitals is a different thing, the abbreviation for estimated time of arrival (American Heritage Dictionary).
Also searched as: eta letter, eta symbol, η, Η, η meaning.
Capital, small and look-alike forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Η | U+0397 | Greek capital letter eta |
| Small | η | U+03B7 | Greek small letter eta |
| Small with accent (tonos) | ή | U+03AE | Greek small letter eta with tonos |
Cambridge Dictionary gives the letter’s symbols as H and η: the capital is shaped like the Latin H but is a separate character, U+0397.
Eta in physics
η is the coefficient of viscosity. OpenStax’s College Physics defines it through the equation F = ηvA/L: the greater the viscosity, the greater the force required.
In AI and machine learning
η is one of the letters for the learning rate, the size of each step in gradient descent. scikit-learn’s guide to stochastic gradient descent calls η “the learning rate which controls the step-size in the parameter space”, which can be constant or gradually decaying. Michael Nielsen’s online textbook Neural Networks and Deep Learning writes each step as Δv = −η∇C, “where η is a small, positive parameter (known as the learning rate)”. Not everyone uses η: Stanford’s CS229 notes call the learning rate α, and PyTorch’s optimisers call it γ.
Write in Greek letters
stringmash.comGreek alphabet translatorWrite English in Greek letters by sound, or turn Greek back into Latin letters as Greek passports do.Read next
- Next in Greek lettersThetaTheta (Θ, θ) is the eighth letter of the Greek alphabet. θ marks an angle, and in statistics and machine learning it stands for a model’s unknown parameters. In computer science, a function in Θ(g) grows at approximately the same rate as g.
- The same shape in Barcodes and QR codesCode 39Code 39 is an older barcode for letters and numbers, used on car parts labels and US military shipping labels. Its text is often printed between two asterisks, which mark the start and end of the code.
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek and Coptic, pp. 39–41
- eta (Cambridge Dictionary entry), Cambridge Advanced Learner’s Dictionary & Thesaurus, Cambridge University Press, eta, noun
- eta (American Heritage Dictionary entry), The American Heritage Dictionary of the English Language, Fifth Edition, HarperCollins Publishers, ETA, abbr.; eta, n.
- Modern Greek Alphabet And Phonetics, University of Arizona, Center for Middle Eastern Studies, Letter table; sounds written several ways
- College Physics 2e, 12.4 Viscosity and Laminar Flow; Poiseuille’s Law, OpenStax (Rice University), Equation 12.41
- 1.5. Stochastic Gradient Descent, scikit-learn User Guide, scikit-learn developers, 1.5.8 Mathematical formulation
- Neural Networks and Deep Learning, chapter 1: Using neural nets to recognize handwritten digits, Michael Nielsen (online textbook), Determination Press, Learning with gradient descent
- CS229 Lecture Notes, Andrew Ng and Tengyu Ma, Stanford University, Part I, section 1.1 (LMS algorithm)
- torch.optim.SGD, PyTorch 2.14 documentation, PyTorch Foundation, Algorithm
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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