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 (∇)).
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 lettersIotaIota (Ι, ι) is the ninth letter of the Greek alphabet. In English “iota” also means an extremely small amount, as in “not one iota of evidence”, and the word “jot” comes from it.
- The same shape in AI labels and iconsAI iconAI-generated or AI-changed content. A badge with the letters AI, often in a circle or beside a word such as generated or modified, labels an image, video, sound or text as made or altered with artificial intelligence. The European Union publishes an official set of these icons.
More greek letters
The same shape elsewhere
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
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
Added .