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**Theta** ( UK: /ˈθiːtə/ , US: /ˈθeɪtə/ ; uppercase **Θ** or **ϴ**, lowercase **θ** (which resembles digit ** 0 ** with horizontal line) or **ϑ**; Ancient Greek : θῆτα *thē̂ta* [tʰɛ̂ːta] ; Modern: θήτα *thī́ta* [ˈθita] ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth

In Ancient Greek, θ represented the aspirated voiceless dental plosive /t̪ʰ/, but in Modern Greek it represents the voiceless dental fricative /θ/.

In its archaic form, θ was written as a cross within a circle (as in the Etruscan

The cursive form ϑ was retained by Unicode as U+03D1 ϑ "GREEK THETA SYMBOL" ("=script theta"), separate from U+03B8 θ "GREEK SMALL LETTER THETA". For the purpose of writing Greek text, the two can be font variants of a single character, but θ and ϑ are also used as distinct symbols in technical and mathematical contexts.

In Latin script used for the Gaulish language, theta developed into the *tau gallicum*, conventionally transliterated as Ð (eth), although the bar extends across the centre of the letter. The phonetic value of the *tau gallicum* is thought to have been [t͡s]. In March, 2020 Unicode added four characters to represent *tau gallicum*:^{ [1] }

- U+A7C7ꟇLATIN CAPITAL LETTER D WITH SHORT STROKE OVERLAY
- U+A7C8ꟈLATIN SMALL LETTER D WITH SHORT STROKE OVERLAY
- U+A7C9ꟉLATIN CAPITAL LETTER S WITH SHORT STROKE OVERLAY
- U+A7CAꟊLATIN SMALL LETTER S WITH SHORT STROKE OVERLAY

The early Cyrillic letter * fita * (Ѳ, ѳ) developed from θ. This letter existed in the Russian alphabet until the 1918 Russian orthography reform.

In the International Phonetic Alphabet (IPA), [θ] represents the voiceless dental fricative, as in * thick* or

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The lowercase letter θ is used as a symbol for:

- A plane angle in geometry
- An unknown variable in trigonometry
- A special function of several complex variables
- One of the Chebyshev functions in prime number theory
- 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 statistical parameter frequently used in writing the likelihood function
- The Watterson estimator for the population mutation rate in population genetics
- Indicates a minimum optimum integration level determined by the intersection of GG and LL schedules (The GG-LL schedules are tools used in analyzing the potential benefits of a country pegging their domestic currency to a foreign currency.)
- The reserve ratio of banks in economic models
- The ordinal collapsing function developed by Solomon Feferman
^{ [2] } - Heaviside step function
- In pharmacology, the fraction of ligand bound to a macromolecule based on the Hill Equation

The uppercase letter Θ is used as a symbol for:

- Dimension of temperature, by SI standard (in italics)
- Dimensionless temperature in transport phenomena
- An asymptotically tight bound in the analysis of algorithms (big O notation)
- A certain ordinal number in set theory
- Pentaquarks, exotic baryons in particle physics
- A brain signal frequency (beta, alpha, theta, delta) ranging from 4–8 Hz
- One of the variables known as "Greeks" in finance, representing time decay of options or the change in the intrinsic value of an option divided by the number of days until the option expires
- A variable indicating temperature difference in heat transfer
- Measuring the angle of incident X-ray beam during XRD

- In ancient times, Tau was used as a symbol for life or resurrection, whereas the eighth letter of the Greek alphabet, theta, was considered the symbol of death.

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*).^{ [3] }

The Egyptians also used the symbol of a point within a circle (*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,^{[ citation needed ]} 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.^{ [3] } 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.^{ [4] } For this reason, use of the number theta was sometimes avoided where the connotation was felt to be unlucky—the mint marks of some Late Imperial Roman coins famously have the sum ΔΕ or ΕΔ (delta and epsilon, that is 4 and 5) substituted as a euphemism where a Θ (9) would otherwise be expected.

- Greek Theta

Preview | Θ | θ | ϑ | ϴ | ᶿ | |||||
---|---|---|---|---|---|---|---|---|---|---|

Unicode name | GREEK CAPITAL LETTER THETA | GREEK SMALL LETTER THETA | GREEK THETA SYMBOL | GREEK CAPITAL THETA SYMBOL | MODIFIER LETTER SMALL THETA | |||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 920 | U+0398 | 952 | U+03B8 | 977 | U+03D1 | 1012 | U+03F4 | 7615 | U+1DBF |

UTF-8 | 206 152 | CE 98 | 206 184 | CE B8 | 207 145 | CF 91 | 207 180 | CF B4 | 225 182 191 | E1 B6 BF |

Numeric character reference | Θ | Θ | θ | θ | ϑ | ϑ | ϴ | ϴ | ᶿ | ᶿ |

Named character reference | Θ | θ | ϑ, ϑ, ϑ | |||||||

DOS Greek | 135 | 87 | 159 | 9F | ||||||

DOS Greek-2 | 172 | AC | 226 | E2 | ||||||

Windows 1253 | 200 | C8 | 232 | E8 | ||||||

TeX | \Theta | \theta | \vartheta |

- Coptic Thethe

Preview | Ⲑ | ⲑ | ||
---|---|---|---|---|

Unicode name | COPTIC CAPITAL LETTER THETHE | COPTIC SMALL LETTER THETHE | ||

Encodings | decimal | hex | decimal | hex |

Unicode | 11408 | U+2C90 | 11409 | U+2C91 |

UTF-8 | 226 178 144 | E2 B2 90 | 226 178 145 | E2 B2 91 |

Numeric character reference | Ⲑ | Ⲑ | ⲑ | ⲑ |

- Cyrillic Fita

Preview | Ѳ | ѳ | ||
---|---|---|---|---|

Unicode name | CYRILLIC CAPITAL LETTER FITA | CYRILLIC SMALL LETTER FITA | ||

Encodings | decimal | hex | decimal | hex |

Unicode | 1138 | U+0472 | 1139 | U+0473 |

UTF-8 | 209 178 | D1 B2 | 209 179 | D1 B3 |

Numeric character reference | Ѳ | Ѳ | ѳ | ѳ |

- Mathematical Theta

Preview | 𝚯 | 𝛉 | 𝚹 | 𝛝 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL BOLD CAPITAL THETA | MATHEMATICAL BOLD SMALL THETA | MATHEMATICAL BOLD CAPITAL THETA SYMBOL | MATHEMATICAL BOLD THETA SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120495 | U+1D6AF | 120521 | U+1D6C9 | 120505 | U+1D6B9 | 120541 | U+1D6DD |

UTF-8 | 240 157 154 175 | F0 9D 9A AF | 240 157 155 137 | F0 9D 9B 89 | 240 157 154 185 | F0 9D 9A B9 | 240 157 155 157 | F0 9D 9B 9D |

UTF-16 | 55349 57007 | D835 DEAF | 55349 57033 | D835 DEC9 | 55349 57017 | D835 DEB9 | 55349 57053 | D835 DEDD |

Numeric character reference | 𝚯 | 𝚯 | 𝛉 | 𝛉 | 𝚹 | 𝚹 | 𝛝 | 𝛝 |

Preview | 𝛩 | 𝜃 | 𝛳 | 𝜗 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL ITALIC CAPITAL THETA | MATHEMATICAL ITALIC SMALL THETA | MATHEMATICAL ITALIC CAPITAL THETA SYMBOL | MATHEMATICAL ITALIC THETA SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120553 | U+1D6E9 | 120579 | U+1D703 | 120563 | U+1D6F3 | 120599 | U+1D717 |

UTF-8 | 240 157 155 169 | F0 9D 9B A9 | 240 157 156 131 | F0 9D 9C 83 | 240 157 155 179 | F0 9D 9B B3 | 240 157 156 151 | F0 9D 9C 97 |

UTF-16 | 55349 57065 | D835 DEE9 | 55349 57091 | D835 DF03 | 55349 57075 | D835 DEF3 | 55349 57111 | D835 DF17 |

Numeric character reference | 𝛩 | 𝛩 | 𝜃 | 𝜃 | 𝛳 | 𝛳 | 𝜗 | 𝜗 |

Preview | 𝜣 | 𝜽 | 𝜭 | 𝝑 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL BOLD ITALIC CAPITAL THETA | MATHEMATICAL BOLD ITALIC SMALL THETA | MATHEMATICAL BOLD ITALIC CAPITAL THETA SYMBOL | MATHEMATICAL BOLD ITALIC CAPITAL THETA SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120611 | U+1D723 | 120637 | U+1D73D | 120621 | U+1D72D | 120657 | U+1D751 |

UTF-8 | 240 157 156 163 | F0 9D 9C A3 | 240 157 156 189 | F0 9D 9C BD | 240 157 156 173 | F0 9D 9C AD | 240 157 157 145 | F0 9D 9D 91 |

UTF-16 | 55349 57123 | D835 DF23 | 55349 57149 | D835 DF3D | 55349 57133 | D835 DF2D | 55349 57169 | D835 DF51 |

Numeric character reference | 𝜣 | 𝜣 | 𝜽 | 𝜽 | 𝜭 | 𝜭 | 𝝑 | 𝝑 |

Preview | 𝝝 | 𝝷 | 𝚹 | 𝞋 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL SANS-SERIF BOLD CAPITAL THETA | MATHEMATICAL SANS-SERIF BOLD SMALL THETA | MATHEMATICAL SANS-SERIF BOLD CAPITAL THETA SYMBOL | MATHEMATICAL SANS-SERIF BOLD THETA SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120669 | U+1D75D | 120695 | U+1D777 | 120505 | U+1D6B9 | 120715 | U+1D78B |

UTF-8 | 240 157 157 157 | F0 9D 9D 9D | 240 157 157 183 | F0 9D 9D B7 | 240 157 154 185 | F0 9D 9A B9 | 240 157 158 139 | F0 9D 9E 8B |

UTF-16 | 55349 57181 | D835 DF5D | 55349 57207 | D835 DF77 | 55349 57017 | D835 DEB9 | 55349 57227 | D835 DF8B |

Numeric character reference | 𝝝 | 𝝝 | 𝝷 | 𝝷 | 𝚹 | 𝚹 | 𝞋 | 𝞋 |

Preview | 𝞗 | 𝞱 | 𝜭 | 𝟅 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL THETA | MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL THETA | MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL THETA SYMBOL | MATHEMATICAL SANS-SERIF BOLD ITALIC THETA SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120727 | U+1D797 | 120753 | U+1D7B1 | 120621 | U+1D72D | 120773 | U+1D7C5 |

UTF-8 | 240 157 158 151 | F0 9D 9E 97 | 240 157 158 177 | F0 9D 9E B1 | 240 157 156 173 | F0 9D 9C AD | 240 157 159 133 | F0 9D 9F 85 |

UTF-16 | 55349 57239 | D835 DF97 | 55349 57265 | D835 DFB1 | 55349 57133 | D835 DF2D | 55349 57285 | D835 DFC5 |

Numeric character reference | 𝞗 | 𝞗 | 𝞱 | 𝞱 | 𝜭 | 𝜭 | 𝟅 | 𝟅 |

These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

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

- Ѳ, ѳ—Fita, a letter of the early Cyrillic alphabet derived from the Greek theta
- ʘ—Bilabial click
- Voiceless dental fricative
- Theta nigrum

- ↑ Everson, Michael; Lilley, Chris (2019-05-26). "L2/19-179: Proposal for the addition of four Latin characters for Gaulish" (PDF).
- ↑ http://math.stanford.edu/~feferman/papers/id-saga.pdf
- 1 2 Barry, Kieren (1999),
*The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World*, Samuel Weiser, p. 73, ISBN 1-57863-110-6 . - ↑ Kaplan, Robert (2000),
*The Nothing that Is: A Natural History of Zero*, Oxford; New York: Oxford University Press, p. 66, Bibcode:2000tnti.book.....K, ISBN 0-19-514237-3 .

**Delta** is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter dalet 𐤃, Letters that come from delta include Latin D and Cyrillic Д.

**Eth** is a letter used in Old English, Middle English, Icelandic, Faroese, and Elfdalian. It was also used in Scandinavia during the Middle Ages but was subsequently replaced with *dh* and later *d*. It is often transliterated as *d*. The lowercase version has been adopted to represent a voiced dental fricative in the International Phonetic Alphabet.

**Gamma** is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop. In Modern Greek, this letter represents either a voiced velar fricative or a voiced palatal fricative.

**H** or **h** is the eighth letter in the ISO basic Latin alphabet. Its name in English is *aitch*, or regionally *haitch*.

**S** or **s** is the 19th letter in the Modern English alphabet and the ISO basic Latin alphabet. Its name in English is *ess*, plural *esses*.

**T** or **t** is the 20th letter in the modern English alphabet and the ISO basic Latin alphabet. Its name in English is *tee*, plural *tees*. It is derived from the Semitic letters taw via the Greek letter τ (tau). In English, it is most commonly used to represent the voiceless alveolar plosive, a sound it also denotes in the International Phonetic Alphabet. It is the most commonly used consonant and the second most common letter in English-language texts.

**Thorn** or **þorn** is a letter in the Old English, Gothic, Old Norse, Old Swedish, and modern Icelandic alphabets, as well as some dialects of Middle English. It was also used in medieval Scandinavia, but was later replaced with the digraph *th,* except in Iceland, where it survives. The letter originated from the rune ᚦ in the Elder Fuþark and was called *thorn* in the Anglo-Saxon and *thorn* or *thurs* in the Scandinavian rune poems. It is similar in appearance to the archaic Greek letter sho (ϸ), although the two are historically unrelated.

**Chi** is the 22nd letter of the Greek alphabet.

**Phi** is the 21st letter of the Greek alphabet.

**Sigma** is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase **∑** is used as an operator for summation. When used at the end of a letter-case word, the final form (ς) is used. In *Ὀδυσσεύς* (Odysseus), for example, note the two lowercase sigmas (σ) in the center of the name and the word-final sigma (ς) at the end.

The **Greek alphabet** has been used to write the Greek language since the late ninth or early eighth century BC. It is derived from the earlier Phoenician alphabet, and was the first alphabetic script in history to have distinct letters for vowels as well as consonants. In Archaic and early Classical times, the Greek alphabet existed in many local variants, but, by the end of the fourth century BC, the Euclidean alphabet, with twenty-four letters, ordered from alpha to omega, had become standard and it is this version that is still used to write Greek today. These twenty-four letters are: Α α, Β β, Γ γ, Δ δ, Ε ε, Ζ ζ, Η η, Θ θ, Ι ι, Κ κ, Λ λ, Μ μ, Ν ν, Ξ ξ, Ο ο, Π π, Ρ ρ, Σ σ/ς, Τ τ, Υ υ, Φ φ, Χ χ, Ψ ψ, and Ω ω.

**Fita** is a letter of the Early Cyrillic alphabet. The shape and the name of the letter are derived from the Greek letter theta (Θ θ). In the ISO 9 system, Ѳ is romanized using F grave accent (F̀ f̀).

Greek letters are used in mathematics, science, engineering, and other areas where mathematical notation is used as symbols for constants, special functions, and also conventionally for variables representing certain quantities. In these contexts, the capital letters and the small letters represent distinct and unrelated entities. Those Greek letters which have the same form as Latin letters are rarely used: capital A, B, E, Z, H, I, K, M, N, O, P, T, Y, X. Small ι, ο and υ are also rarely used, since they closely resemble the Latin letters i, o and u. Sometimes font variants of Greek letters are used as distinct symbols in mathematics, in particular for ε/ϵ and π/ϖ. The archaic letter digamma (Ϝ/ϝ/ϛ) is sometimes used.

**Ŧ**, known as **T with a bar** or **T with a stroke sign**, is the 25th letter in the Northern Sámi alphabet, where it represents the voiceless dental fricative. In the SENĆOŦEN alphabet, it also represents a [θ]. It is also used in the Havasupai alphabet, and to represent the letter ט in romanizations of Hebrew. The Unicode codepoints for this letter are U+0166ŦLATIN CAPITAL LETTER T WITH STROKE and U+0167ŧLATIN SMALL LETTER T WITH STROKE.

**Theta** is the eighth Greek letter, written *Θ* (uppercase) or *θ* (lowercase).

**Th** is a digraph in the Latin script. It was originally introduced into Latin to transliterate Greek loan words. In modern languages that use the Latin alphabet, it represents a number of different sounds. It is the most common digraph in order of frequency in the English language.

**Oe** or **barred O** is a letter of the Cyrillic script.

**Ø** is a Scandinavian vowel letter.

**The** is a letter of the Cyrillic script. The name *the* is pronounced [θɛ], like the pronunciation of ⟨the⟩ in "**the**ft". In Unicode, this letter is called "Es with descender". In Chuvashia, it looks identical to the Latin letter C with cedilla. Occasionally it also has the hook diacritic curved rightward like an ogonek, as in the SVG image shown in the sidebar. In many fonts, the character hooks to the left.

The null sign U+2205∅EMPTY SET denotes the empty set in mathematics. The same letter in linguistics represents zero, the lack of an element. It is commonly used in phonology, morphology, and syntax.

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