Tau (upper case Τ, lower case τ) is the 19th letter of the Greek alphabet. In the system of Greek numerals it has a value of 300.

The lower-case letter τ is used as a symbol for:

• Torque, the rotational force in mechanics.
• The elementary tauon particle in particle physics.
• The lifetime of a spontaneous emission process.
• The time constant in an RC circuit.
• A coefficient of correlation — see Kendall's tau .
• The Golden ratio 1.618... (although φ (phi) is more common)

• Tau in astronomy is a measure of opacity, or how much sunlight cannot penetrate the atmosphere
• Tau is the name of an island near Kilinailau , Greenwich Island , and Ocean Island.
• In Games Workshop's Warhammer 40,000 science fiction gaming universe, the Tau are an alien race.
• Tau is a symbol used from antiquity and is represented by a type of cross. There are those who accuse Christ of being the pirate of this ancient symbol, but it did not become a symbol of Christianity until a later date. As with Christianity, other ancient societies who used the "Tau" symbol also evolved its use to represent resurrection, reincarnation and blood sacrifice.

Tau is the name of a small village near Stavanger in Rogaland, Norway. You travel through Tau to get to the famous Pulpit Rock.