# tau

/tˈaʊ/ (UK) · CEFR C2 · Noun

Source: https://englishreference.com/word/tau
Updated: 2026-05-31

## Definition

the 19th letter of the Greek alphabet. In science and math, it often represents a specific number or a type of tiny particle.

For teachers: the nineteenth letter of the Greek alphabet, represented as τ. In physics, it denotes a specific elementary particle (the tau lepton), while in mathematics, it is frequently used to represent the circle constant equal to 2π.

## Examples

- The scientist used the symbol tau in her equation.
- In many modern geometry circles, mathematicians prefer using tau instead of pi because it represents a full turn around a circle.
- The discovery of the tau lepton provided crucial evidence for the third generation of fundamental particles in the Standard Model of particle physics.

## Usage notes

- When referring to the mathematical constant, it is often treated as an uncountable abstract concept.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.