Theta
Theta (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.
How to read it
Theta is said THEE-tuh in Britain and THAY-tuh in America (Cambridge Dictionary: /ˈθiː.tə/ and /ˈθeɪ.t̬ə/), with the th of “think”. Modern Greek θήτα, “Thita” in the University of Arizona’s guide, has the same unvoiced th.
Also searched as: theta letter, theta symbol, θ, Θ, θ meaning.
Capital, small and look-alike forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Θ | U+0398 | Greek capital letter theta |
| Small | θ | U+03B8 | Greek small letter theta |
| Script theta | ϑ | U+03D1 | Greek theta symbol |
| Capital theta symbol | ϴ | U+03F4 | Greek capital theta symbol |
Unicode describes the script theta ϑ as used as a technical symbol. It cross-refers θ to two look-alikes: the Latin small letter barred o ɵ (U+0275) and the Cyrillic fita ѳ (U+0473).
Theta in maths and statistics
θ marks an angle: in OpenStax’s University Physics, the angle θ that a rotating particle’s position vector sweeps out is its angular position. In statistics θ is the unknown parameter of a model; Jeff Miller’s history of mathematical symbols says R. A. Fisher established that use in 1922, having already used it in a paper he wrote as an undergraduate in 1912. In computer science the capital appears in “Big-Theta” notation: f ∈ Θ(g) says f grows at least as fast and at least as slow as g, a tight bound (Carnegie Mellon University course notes).
In AI and machine learning
θ is the model’s parameters, the weights training adjusts. Stanford’s CS229 notes write a linear model as h_θ(x) = θ₀ + θ₁x₁ + θ₂x₂ and call the θs “the parameters (also called weights)”; gradient descent then updates each θⱼ by the learning rate α times the slope of the cost. PyTorch’s optimiser documentation uses the same letter, with θ₀ for the starting parameters and ∇_θ for the gradient with respect to them (Nabla (∇)).
More greek letters
Browse symbols drawn with circles or 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
- theta (Cambridge Dictionary entry), Cambridge Advanced Learner’s Dictionary & Thesaurus, Cambridge University Press, theta, noun
- theta (American Heritage Dictionary entry), The American Heritage Dictionary of the English Language, Fifth Edition, HarperCollins Publishers, theta, n.
- Modern Greek Alphabet And Phonetics, University of Arizona, Center for Middle Eastern Studies, Letter table
- University Physics Volume 1, 10.1 Rotational Variables, OpenStax (Rice University), Angular position
- Earliest Uses of Symbols in Probability and Statistics, Jeff Miller, Earliest Uses of Various Mathematical Symbols, MacTutor History of Mathematics, University of St Andrews, Notation for parameters and estimates
- 15-210 Recitation 2: Big-O, Big-Omega, and Big-Theta, Carnegie Mellon University, School of Computer Science, Section 2, Definition 2.3
- CS229 Lecture Notes, Andrew Ng and Tengyu Ma, Stanford University, Part I, chapter 1
- torch.optim.SGD, PyTorch 2.14 documentation, PyTorch Foundation, Algorithm
Last reviewed September 30, 2026 by Rory Hansen. How we research