Tau 'tɔː,_'taʊ (uppercase Τ, lowercase τ, or ; ταυ taf) is the nineteenth letter of the Greek alphabet, representing the voiceless dental or alveolar plosive t. In the system of Greek numerals, it has a value of 300.
The name in English is pronounced taʊ or tɔː, but in Greek it is taf. This is because the pronunciation of the combination of Greek letters αυ can have the pronunciation of either [ai], [av] or [af], depending on what follows and if a diaeresis is present on the second vowel (see Greek orthography).
Tau was derived from the Phoenician letter taw (𐤕). Letters that arose from tau include Roman T and Cyrillic Te (Т, т).
The letter occupies the Unicode slots U+03C4 (lowercase) and U+03A4 (uppercase). In HTML, they can be produced with named entities (τ and Τ), decimal references (τ and Τ), or hexadecimal references (τ and Τ).
Greek letters used in mathematics, science, and engineering#Ττ (tau)
The lower-case letter τ is used as a symbol for:
Specific tax amount
The expressed period of the freerunning rhythm of an animal, i.e., the length of the daily cycle of an animal when kept in constant light or constant darkness
The dose interval in pharmacokinetics
The core variable in general tau theory
Tau in biochemistry, a protein associated with microtubules and implicated in neurodegenerative diseases such as Alzheimer's disease, some forms of frontotemporal lobar degeneration, and chronic traumatic encephalopathy
Divisor function in number theory, also denoted d or σ0
Golden ratio (1.618...), although φ (phi) is more common
Kendall tau rank correlation coefficient in statistics
Stopping time in stochastic processes.
Tau, the ratio of the circumference to the radius of a circle, which is equal to 2pi (6.28318...