Theta (upper case Θ, lower case θ) is the eighth letter of the Greek alphabet. In the system of Greek numerals it has a value of 9. Theta Θ represents the phonetical sound of "Th" in English, such as in the words "thick" or "thin".

In its archaic form, theta was written as:

• a cross within a circle or
  
• and later, as a line or point within a circle .
  

According to Porphyry of Tyros, the Egyptians used an X within a circle as a symbol of the soul; having a value of nine, it was used as a symbol for Ennead. Johannes Lydus says that the Egyptians used a symbol for Kosmos in the form of theta, with a fiery circle representing the world, and a snake spanning the middle representing Agathos Daimon.

The Egyptians also used the symbol of a point within a circle to represent the Sun god Ra, which might be a possible origin of its use as the astrological glyph for the Sun. It is worthwhile to note that theta has the same numerical value in isopsephy as Helios (ΘΗΤΑ = 318 = ΗΛΙΟΣ).

In classical Athens, as an abbreviation for the Greek word thanatos (θάνατος: "death"; see Thanatos and Thanatology) and as it vaguely resembles a human skull, theta was used as a warning symbol of death, in the same way that skull and crossbones are used in modern times. It survives on potsherds used by Athenians when voting for the death penalty.

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

• A plane angle in geometry.
  
• θ is the voiceless (inter)dental fricative in the International Phonetic Alphabet.
  

The upper-case letter Θ is used as a symbol for:

• Earth Day
  
• Dimensionless Temperature in transport phenomena
  
• denoting an asymptotically tight bound in Analysis of algorithms. See Big O notation.
  

The Cyrillic letter Fita arose from Theta.

References

The Greek Qabalah, by Kieren Barry, Samuel Weiser, Inc., 1999. ISBN 1578631106

Jesus Christ, Sun of God, by David Fideler, Quest Books, 1993. ISBN 0835606961