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 symbolism of the cross was connected not only to the letter chi, but also to tau, the equivalent of the last letter in the Phoenician and Old Hebrew alphabets, and which was originally cruciform in shape.

An essay written around 160 CE, attributed to Lucian, a mock legal prosecution called The Consonants at Law - Sigma v. Tau in the Court of Seven Vowels contains a reference to the attribution. Sigma petitions the court to sentence Tau to death by crucifixion, saying:

Men weep, and bewail their lot, and curse Cadmus with many curses for introducing Tau into the family of letters; they say it was his body that tyrants took for a model, his shape that they imitated, when they set up structures on which men are crucified. Stauros (cross) the vile engine is called, and it derives its vile name from him. Now, with all these crimes upon him, does he not deserve death, nay, many deaths? For my part I know none bad enough but that supplied by his own shape - that shape which he gave to the gibbet named stauros after him by men.

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)
  
• Ramanujan's tau function in number theory.
  
• Tau in astronomy is a measure of opacity, or how much sunlight cannot penetrate the atmosphere
  
• Tau in biochemistry is a protein associated to microtubules
  
  

Other uses of Tau:

• In ancient times, a symbol for life and/or resurrection, whereas the 8th letter of the Greek alphabet, theta, was considered the symbol of death; many say that these two symbols created today’s plus (+) and minus (-) symbols
  
• In Biblical times, the Tau cross was put on men to distinguish those who lamented sin, although newer versions of the Bible have replaced the ancient term “Tau” with “mark” (Ezekiel 9:4)
  
• Tau is the name of an island near Kilinailau, Greenwich Island, and Ocean Island.
  
• Tau is the name of a small town in western Norway.
  
• In Games Workshop's Warhammer 40,000 science fiction gaming universe, the Tau are an alien race.
  
• Cross of Tau
  
• The prefix of many cosmic entities, fictional or not. Notably Tau Ceti Five from the computer game System Shock 2