Theta
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2008-07-08T13:00:20Z
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/* Usage in names */
{{wiktionarypar2|Θ|θ}}
{{Table Greekletters|letter=theta}}
{{dablink|For other senses of this word, see [[theta (disambiguation)]].}}
'''Theta''' (uppercase '''Θ''', lowercase '''θ''' or '''{{IPA|ϑ}}'''; {{lang-el|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 [[aspiration (phonetics)|aspirated]] [[voiceless dental plosive]] ({{IPA|/t̪ʰ/}}), but in [[Koiné]] and later dialects it became a [[voiceless dental fricative]] {{IPA|/θ/}}. In American English the name of the letter is commonly {{pronEng|ˈθeɪtʌ}}. In [[British English]], the name is pronounced {{IPA|/ˈθiːtə/}}.
Letters such as the early [[Cyrillic]] letter ''[[fita]]'' ({{Unicode|Ѳ, ѳ}}) developed from θ.
In its [[Archaic period in Greece|archaic]] form, θ was written as:
* a cross within a circle ([[Image:Earth symbol.svg|16px|A symbol of a cross within a circle]] or [[Image:Cross within circle 2.png|16px|Another symbol of a cross within a circle]])
* and later, as a line or point within a circle ([[Image:Line within circle.png|16px|The symbol of a line within a circle]] or [[Image:Sun symbol.svg|16px|The symbol of a point within a circle]]).
According to [[Porphyry of Tyros]], the [[Ancient Egypt|Egyptian]]s 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'').<ref name="Barry_1">{{Harvard reference| Author=Barry, Kieren
| Last=Barry
| First=Kieren
| Title=The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World
| Publisher=[[Samuel Weiser]]
| page=p. 73
| Year=1999
| ID=ISBN 1-57863-110-6
}}.</ref>
The Egyptians also used the symbol of a point within a circle ([[Image:Sun symbol.svg|15px|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 {{Polytonic|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 language|Greek]] θάνατος (''thanatos'', “death”) and as it vaguely resembles a human skull, theta was used as a warning symbol of [[Thanatos|death]], in the same way that [[skull and crossbones]] are used in modern times. It survives on [[potsherd]]s used by Athenians when voting for the [[death penalty]].<ref name="Barry_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.<ref>{{Harvard reference
| last = Kaplan
| first = Robert
| title = The Nothing that Is: A Natural History of Zero
| publisher = [[Oxford University Press]]
| date = 2000
| pages = p. 66
| isbn = 0-1951-4237-3
}}.</ref>
==Symbol==
The lower-case letter θ is used as a symbol for:
* A [[angle|plane angle]] in geometry.
* The [[voiceless dental fricative]] in the [[International Phonetic Alphabet]], such as in the words ''<u>th</u>ick'' or ''<u>th</u>in'' (but not as in ''<u>th</u>e'', 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.
* 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.{{Fact|date=October 2007}}
* The parameter in the [[likelihood function]]
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 [[pentaquark]]s, exotic [[baryon]]s in [[particle physics]].
* [[Earth Day]].
* [[Electroencephalography|Brain Signal Frequency]] (Beta, Alpha, Theta, Delta) ranging from 4–8 Hz
* One of the variables known as [[Greeks (finance)]], representing the lost-value rate of an option over time.
==Usage in names==
* [[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]].<!-- Name of band? -->
* Theta is the name of a biological weapon and boss featured in the Atari video game [[Area 51 (2005 video game)|Area 51]].
* Thetamen is a nickname also used by the [[Fraternities and sororities|fraternity]] [[Iota Phi Theta]].
* Theta Phi Alpha is a sorority.
* Theta refers to a member of the Alpha Pi Theta Brotherhood of [[Emerson College]]
* [http://www.thetahealing.com/ Theta healing] is a name for healing procedure where practitioner and client enter theta brainwawe frequency.
==References==
<references/>
[[Category:Greek letters]]
[[Category:Phonetic transcription symbols]]
[[als:Θ]]
[[ar:ثيتا (حرف إغريقي)]]
[[arc:Θ]]
[[ast:Theta]]
[[br:Theta (lizherenn)]]
[[ca:Theta]]
[[cy:Theta]]
[[da:Theta]]
[[de:Theta]]
[[el:Θήτα]]
[[es:Θ]]
[[eo:Teto]]
[[ext:Teta]]
[[eu:Thita]]
[[fr:Thêta]]
[[ga:Téite]]
[[gd:Theta]]
[[gl:Theta]]
[[ko:Θ]]
[[hr:Theta]]
[[is:Þeta]]
[[it:Theta (lettera)]]
[[he:תטא]]
[[ka:თეტა (ასო)]]
[[ht:Θ]]
[[la:Theta]]
[[hu:Théta]]
[[ms:Teta]]
[[nah:Θ]]
[[nl:Thèta]]
[[ja:Θ]]
[[no:Theta]]
[[nn:Theta]]
[[nds:Theta]]
[[pl:Theta]]
[[pt:Θ]]
[[ru:Тета (буква)]]
[[simple:Theta]]
[[sk:Théta]]
[[sl:Theta]]
[[sh:Theta]]
[[fi:Theeta]]
[[sv:Theta]]
[[th:ทีตา]]
[[tr:Θ]]
[[uk:Тета (літера)]]
[[zh:Θ]]