theta

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Look up Θ or θ in Wiktionary, the free dictionary.

Greek alphabet
Αα	Alpha	Νν	Nu
Ββ	Beta	Ξξ	Xi
Γγ	Gamma	Οο	Omicron
Δδ	Delta	Ππ	Pi
Εε	Epsilon	Ρρ	Rho
Ζζ	Zeta	Σσς	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
	Digamma		Qoppa
	San		Sampi
Other characters
	Stigma		Sho
	Heta
Greek diacritics

Theta (uppercase Θ, lowercase θ or ϑ; Greek: Θήτα) 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, it 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] Petrus de Dacia in a document from 1291 relates the idea that theta was used to brand criminals as empty ciphers, and the branding rod was affixed to the crossbar spanning the circle.^[2]

Symbol

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

• A plane angle in geometry.
• A special function of several complex variables.
• 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.
• Threshold value of an artificial neuron.
• A Bayer designation letter applied to a star in a constellation; usually the eighth star so labelled but not necessarily the eighth brightest as viewed from Earth.
• The parameter frequently used in writing the likelihood function (Bayes theorem).
• Along with μ (Mu), population parameters in the Normal/Gaussian/Bell curve distribution, usually designated N(μ,θ).

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.
• One of the variables known as "Greeks" in finance, representing the lost-value rate of an option over time.
• As a variable indicating Temperature difference in Heat Transfer

The common usage of theta in mathematical problems began with the Greeks. They logically chose this Greek symbol since it was the next in their alphabet not yet used and was easily legible when recreating for multiple uses. Archaic crossed forms of theta are seen in the wheel letters of Linear A, Linear B, and Egyptian hieroglyphics, the mathematical tensor product, exclusive disjunction, and direct sum operators, the astrological symbol for the Earth (as already mentioned), and the Celtic cross. The circumpunct, or circled dot, is seen in a wide range of places.

Usage in names

• Theta Nu Kappa is a Multi-Cultural Academic Fraternity, Founded in 2007 at University of California, Santa Barbara
• 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.
• In generative grammar, theta roles are representations of a constituent's syntactic and semantic contributions to a sentence.
• In the machinima Red vs Blue, Theta is the AI of the Freelancer North Dakota.
• In Scientology, the word "theta" is often used as an adjective to express or convey a sense of spirituality or uplifting feeling. See also thetan, the Scientology term for "spirit".
• Theta Sigma is one of The Doctor's nicknames in the BBC Drama Doctor Who.
• Kappa Alpha Theta is a women's fraternity whose members are designated “Thetas”.
• Theta Phi Alpha is a sorority.
• Theta Sigma Chi is a women's social club at Oklahoma Baptist University
• Theta Sigma Chi is a sorority at Austin College, Sherman, Texas, founded in 1985.
• Phi Delta Theta is a Fraternity
• Theta refers to a member of Sigma Phi Theta, a local fraternity at Texas Lutheran University.
• Thetamen is a nickname also used by the fraternity Iota Phi Theta.
• Theta Chi is a college social fraternity.
• Theta refers to a member of the Alpha Pi Theta Brotherhood of Emerson College.
• Theta Sigma Tau is a local Fraternity at Ripon College in Ripon Wisconsin [1]
• The Theta Society is a non-collegiate social sorority, women ages 35 and older, in Texas.
• In the video game Fallout 3, one of the available radio frequencies is Recon Craft Theta which leads to a crashed UFO. It is mainly used to get to a ship named Mothership Zeta, which is available in the downloadable content of the same name.

Notes

See also

• Fita, a letter of the early Cyrillic alphabet derived from the Greek theta.