Tau
Tau (Τ, τ) is the 19th letter of the Greek alphabet, said “tow” (rhyming with cow) or “taw”. In physics τ is the symbol for torque and for a circuit’s time constant.
How to read it
The Cambridge Dictionary gives /taʊ/, rhyming with “cow”, and Dictionary.com adds “taw”, “tahv” and “tawv”. The Collins English Dictionary describes tau as a consonant, transliterated as t. The capital Τ has the same shape as the Latin T.
Also searched as: tau letter, tau symbol, τ, Τ.
Capital and small forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Τ | U+03A4 | GREEK CAPITAL LETTER TAU |
| Small | τ | U+03C4 | GREEK SMALL LETTER TAU |
Tau in physics and statistics
Torque is written τ (OpenStax University Physics). In an electric circuit with a resistor and a capacitor, the time constant is τ = RC, in seconds: after one time constant a charging capacitor holds 63.2% of its maximum charge (OpenStax). In statistics Kendall’s tau is a correlation measure for rankings; SciPy calculates it as kendalltau. Compare Rho for the ordinary correlation coefficient.
In AI and machine learning
τ is a temperature. PyTorch’s Gumbel-softmax function takes a parameter tau, which its documentation describes as a non-negative scalar temperature.
Other uses
In particle physics τ is the tau, a lepton that OpenStax describes as about 3,500 times heavier than the electron but otherwise similar to it. In biology “tau” is also the name of a protein in nerve cells (Dictionary.com), written as a word, not a symbol.
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 lettersUpsilonUpsilon (Υ, υ) is the 20th letter of the Greek alphabet, a vowel written y or u in Latin letters. In physics Υ is the upsilon meson, a particle made of a bottom quark and its antiquark.
- In Physics and chemistry symbolsHalf-life (t½)t½ (also written t1/2 or T½) is the half-life: the time it takes for half of a radioactive substance to decay, or in chemistry for half of a reactant to be used up.
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- tau (Dictionary.com entry), Dictionary.com Unabridged; Collins English Dictionary, Dictionary.com, Entry, American and British (Collins) senses
- tau (Cambridge Dictionary entry), Cambridge Advanced Learner’s Dictionary, Cambridge University Press, Entry
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek capital and small letters
- University Physics Volume 1, 10.6 Torque, OpenStax (Rice University), Defining torque
- University Physics Volume 2, 10.5 RC Circuits, OpenStax (Rice University), Charging a capacitor
- scipy.stats.kendalltau, SciPy v1.18 manual, SciPy developers, Description
- torch.nn.functional.gumbel_softmax, PyTorch 2.14 documentation, PyTorch Foundation, Parameters
- University Physics Volume 3, 11.1 Introduction to Particle Physics, OpenStax (Rice University), Leptons
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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