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.
More greek letters
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
Last reviewed September 30, 2026 by Rory Hansen. How we research