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Greek alphabet
Αα Alpha	Νν Nu
Ββ Beta	Ξξ Xi
Γγ Gamma	Οο Omicron
Δδ Delta	Ππ Pi
Εε Epsilon	Ρρ Rho
Ζζ Zeta	Σσ Sigma
Ηη Eta	Ττ Tau
Θθ Theta	Υυ Upsilon
Ιι Iota	Φφ Phi
Κκ Kappa	Χχ Chi
Λλ Lambda	Ψψ Psi
Μμ Mu	Ωω Omega
Obsolete letters
Ϝϝ Digamma	Ϸϸ Sho
Ϛϛ Stigma	Ϙϙ Qoppa
Ϻϻ San	Ϡϡ Sampi
Greek diacritics

Theta (uppercase Θ, lowercase θ or ϑ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. In the system of Greek numerals it has a value of 9. In Classical Greek θ represented an aspirated voiceless dental plosive (/t̪ʰ/), but in Koiné and later dialects it became a voiceless dental fricative /θ/. In American English the name of the letter is commonly pronounced /ˈθeɪtə/. In British English, the name is pronounced /ˈθiːtə/.

Letters such as the early Cyrillic letter fita (Ѳ, ѳ) developed from θ.

In its archaic form, θ was written as:

• a cross within a circle ( or )

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

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 (literally: good spirit).^[1]

The Egyptians also used the symbol of a point within a circle (, the sun disc) to represent the sun, which might be a possible origin of its use as the Sun's astrological glyph. It is worthwhile to note that θῆτα (theta) has the same numerical value in isopsephy as Ηλιος (Helios): 318.

In classical Athens, it was used as an abbreviation for the Greek θάνατος (thanatos, “death”) 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.^[1]

Contents 1 Symbol 2 In options investing 3 In popular sociology 4 Usage in names 5 References

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

• A plane angle in geometry.
• The voiceless dental fricative in the International Phonetic Alphabet, such as in the words thick or thin (but not as in the, which is the voiced dental fricative).
• The Potential temperature in meteorology.
• The score of a test taker in item response theory.
• Theta Type Replication: a type of bacterial DNA replication specific to circular chromosomes
• Theta has also gained a new significance, in the north-eastern region of the United States, as a symbol for affection, in a similar capacity to the “heart” symbol.^{[citation needed]}

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

• Dimensionless temperature in transport phenomena.
• Big O notation. Denoting an asymptotically tight bound in analysis of algorithms.
• Θ (set theory), a certain ordinal number.
• Representing pentaquarks, exotic baryons in particle physics.
• Earth Day.
• Brain Signal Frequency (Beta, Alpha, Theta, Delta) ranging from 4–8 Hz

Theta is known as an option Greek measuring the rate of time decay on a daily basis. One can look at this as an ice cube melting in your hand. A Theta of  −.05 means that the option is losing a nickel of time value each day. Other option Greeks include Delta, Vega, Rho and Gamma.^[2]

See also: Greeks (finance)

Theta is used colloquially to refer to the quotient of a person's appearance versus time.^{[citation needed]} Similar to the Options investing as a description of the effect of time on price, Theta in this sense describes the effect of time on appearance. For example, a 50-year old woman, who looks as if she were 40 years old, would have a Theta of 50/40 = 1.20. An excellent example is Vanna White, who appears approximately 10 years younger than her calendar age. Theta is a desirable quality; people generally are more attracted to a person with good theta than those who look their age.

• Theta Sigma is one of the nicknames of the Doctor of the television series Doctor Who.
• Kappa Alpha Theta is a sorority whose members are designated “Thetas”.
• Theta refers to a member of Sigma Phi Theta, a local fraternity at Texas Lutheran University.
• Theta is also the name of a melodic death metal band based in Plattsburgh, New York.
• Theta is the name of a biological weapon and boss featured in the Atari video game Area 51.

1. ^ ^{a b} Barry, Kieren (1999), The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World, Samuel Weiser, p. 73, ISBN 1-57863-110-6.
2. ^ McMillan, Lawrence (2002), Options as a strategic investment, Prentice Hall Press, pp. 848–866, ISBN 0-7352-0197-8.